Short answer. No.
Long answer.
If you are destined to do work to the heads, upgrading the valves is not where your time/money should be spent. The castings are the bottleneck, not the valves. The GT40p heads outflow the GT40 heads, which use a larger exhaust valve. People that have taken the effort to install larger valves in GT40p heads have found, it helped but only just barely...certainly not nearly enough to come close to justifying the cost of the machining, aftermarket valves, and piston modifications to the lashes.
So what can you do?
Porting them will increase their output, but you have to be careful where you port. Porting in the wrong areas of the bowl where it transitions to the compression chamber or in the exhaust port will actually reduce their flow. diyPorting.com has some great info on what and where to port GT40p heads:
Porting GT40p Intake Port
Porting GT40p Exhaust Port
Also note how the documents instruct you to port a hump in the top of the exhaust port:
This is because most people are porting GT40p heads for use on Mustangs. If you put Mustang gaskets on GT40p heads, you can see that there is room to port that hump into them:
But if you are using Explorer equipment, you CANNOT cut the port higher. This is because the Explorer exhaust gaskets line right up with the tops of the Exhaust ports (no pic of that, sorry).
The next thing you'll want to do is back-cut the stock valves. If money were no object you'd replace the valves with aftermarket. But if budget was no object, you'd be upgrading to a set of AFRs. So, from a practical bang-for-the-buck, just reuse the stock valves. Used stock valves will likely have a lip worn into them that'll need to be cut out. Back-cutting that lip off will help them flow a tiny bit better...enough that it's worth doing if you've taken the time to port the heads. A lot of people pay for the 3-angle valve job, but the seats of my GT40p heads were so narrow, the machine shop could only get 2 angles on them. So if you do go for the 3-angle, don't be surprised if you can only get 3-angles on the valves, but you can't get 3-angles on the head's valve seats. GT40p heads use the cast iron of the head as the valve seat as opposed to special inserts. Apparently, Ford flame-hardens the seats to make them durable instead of using a special material. That seems to work OK for them.
Another highly recommended thing to do is have them decked. I CCed my GT40p heads and they were WAY larger compression chambers than the 58ccs GT40p heads were supposed to be. I found mine to be in the 65-66cc range in stock form. Even after decking them down .010", they were still in the 61cc range. So deck them AT LEAST .010" to get the compression a bit higher on them. Decking them down to reduce the compression chamber CCs will improve the compression a mild amount to make the engine a tad more powerful & more fuel efficient.
If you were reusing the stock cam, then there'd be very little need in doing anything with the springs and retainers. However with an aftermarket cam, stock equipment will RPM-limit you. The stock spring retainers are big, HEAVY, and will float the valves quite easily with a more aggressive cam...that's if your aftermarket cam doesn't bottom them out. Aftermarket springs and retainers are readily available and aren't that expensive. Just make sure you get a set made for GT40p heads AND for the lifts your cam will be pushing. Also keep in mind, the stock intake and exhaust valves are NOT identical spring heights. If you don't know what this means, then you definitely need to buy a spring kit from someone that does know what this means. I bought my spring/retainer set from
Alex's Parts and I would gladly recommend them to anybody else. At the time, I looked into modifying the castings to take beehive springs, but unless as part of my research, I found unless you are going to be reving into the 6500+ range on a regular basis, beehive springs are not worth the cost. Stick with the regular aftermarket upgrades that match your cam lift, and you'll be fine. BTW, what is your replacement cam specs?
Finally, when you go back together with the engine, DO NOT use stock OEM Explorer head gaskets. I measured the stock and replacement gaskets out at .047". This is bad because GT40p heads have a beautiful quench pad, but you can't make use of them with a gasket this thick. My stock short block located each piston in the hole .014-.015" (aka deck clearance). That deck clearance combined with the stock .047" gasket produces a quench of well into the .060" range which effectively means you have no quench. Ideal quench is in the .030-.040" range. The benefits of quench are well documented. Just google-search if you want to know more. I won't attempt to repeat what has been said elsewhere, and with far better detail. What's important to note is as quench thickness goes above .040", the benefits of quench begin to diminish. By .060", all benefits have been lost. So to get back what the stock motor never had, you'll want "super-thin" head gaskets that are in the .025" thickness range. The catch is those are rather EXPENSIVE gaskets. They will be easily 4x more expensive than the FelPro OEM Explorer 5.0L replacements. Before you buy any gasket, measure the deck height of each piston in your block while the heads are off and confirm that your pistons are in the hole .015" like mine were. If they are, then you'll want those super-thin gaskets if you want to get the benefits of quench. The thinner gaskets will also increase your compression while REDUCING the engine's tendency to detonate.
For reference, I setup my 331 with pistons that pop out of the block .004-.006". I used a .039" thick head gasket to give me a ~.035" quench. Based on my CCing of the heads, gasket thickness, flat-top pistons, .030" over-bore, and crank stroke, I estimated my Static Compression Ration (SCR) somewhere in the 10.1:1 range. With my very mild Crane cam, this put my Dynamic Compression Ratio (DCR) in the 8.25 range. 8.5 DCR is considered the highest you'd want to go while still using premium pump gas. I didn't want to push the envelope by aiming for 8.5...so I aimed for a value just below. But even with the rather high 8.25:1 DCR, I found I can still run 87 octane. I had to retard the timing on my tune, but it does run regular without excessive ping and I thank that to my attention to preserving the quench.
Of all the things I did to increase compression, the largest contributor was the increased stroke from the 331 stroker kit. Even if you decked the heads, invested in the thinner gasket, ported them, and pulled the block to deck it to get your pistons closer to the deck, you would still be well BELOW the compression range that would require you run premium. Instead what you'll find is the engine is just a tad more peppy, gets better fuel economy, and will run the super-cheap copper plugs without a problem (no need for $4/plug Platinums or $8/plug Iridiums). Being you are replacing the cam, my GUESS is you are going with something that's more aggressive. I wouldn't go too aggressive or you risk dropping your DCR to below what it was with the stock cam and the stock setup. That could loose you some off-the-line torque.
That's about all I know to say about the GT40p heads.
Now what about the Explorer intake? The good news is the Explorer upper/lower is actually a very good intake. The Explorer upper has longer runners than the other GT40-style uppers so it is more torque-oriented but still capable of ~300hp in stock form. Unfortunately there's not much you can do with the upper to make any significant gains. The walls are rather thin and to get any measurable gains from the upper would require mud-honing, which really isn't worth the cost. The lower is where you should focus. There is
ample porting opportunity in the lower. I ported my lower intake and I can tell you from 1st hand experience, there's FAR more material that can be removed from the intake than there is in the heads. Don't get me wrong, porting the heads is worth while. But the lower has some obvious obstructions that can be removed to bump its capacity well into the 325+hp range. If you are interested in this, Tom Moss of TMoss Porting has a document talking about how to port a GT40 lower:
Ported GT40 Intake Dyno & Track Data
One of the things he warns about is welding up the inside knees of the #1 and #5 runner (the two runners at the very front). Again from first hand experience, he ain't jokin' on this. You WILL want to get the backsides of them welded up to thicken them so you can do a better porting job on them. The #1 and #5 are the longest runners and the #5 is the most obstructed due to the sweeping angle it has cast into it. Thus porting all the others and not welding these up thicker so you can do a proper porting job on them is a mistake IMO. I wish I had a good picture of what your welds should look like, but I don't. Best I can tell you is weld a LOT. Fill in the entire area behind both of these ports INCLUDING up the runner stack, not just down at the bottom where it's obvious. It's a whole lot easier to add aluminum while the aluminum is thick than to bust through while porting and have to build the wall up after you've thinned it out...which is exactly what I did because I didn't get the welds thick enough and in the right places the 1st time.