What pistons are you using? You need 25D pistons and it looks like, based on your head gaskets you are using 251 block and gasket thickness. The compression ratio on that combo is too high and it will suffer from severe detonation. The ECM will yank out your timing and kill the performance.
Looks like those heads may have been decked. There shouldn't be a significant fitment issue with more or less stock block, heads, and intake.
If you have plenty of parts to shop then the combo you want is a 255/257 rotating assembly, #5 thrust block, and 25D heads. Shave off about 0.020 on the heads, run 257 gaskets. You will need eccentric idlers for the timing belt, and you will have to enlarge the holes on the 25D intake. Very exceptional power band from that combo.
GD