I too was thinking of the OHC shim gaskets as "normal" as that is what I found when I tore a few down. These are not at all stiff, they could be easily formed and also deformed just by lifting them off the block. And they are certainly well-compressed!

I also see a substantial difference in one hole in the Fel-Pro set. Could/should the block passage below be opened up to match the newer gasket? (On a non-matching block, of course )

I've also seen some replacement pistons with noticeable bevels at the top corners, unlike the stock castings.

I am interested in knowing who is running which pistons (brand and part #) on a 400, if you had your block decked, which head gaskets you are using, and if your combination is fine on performance and temperature.