The ‘604 crate engine has an excellent intake manifold and cylinder for this application. Porting for air flow improvement on these heads and intake for more performance is a waste of time when running the stock lift camshaft. So, instead of spending money on head and intake manifold work for the ‘604 crate, go buy a new set of tires. If you want a significant performance improvement change the camshaft.
The Fast Burn, Vortec Bow-Tie style aluminum heads on the ‘604 are excellent flowing stock heads.
Unlike the heads on the ‘602 & ‘603 crate motors, which improve tremendously with “UnderCover Porting“, this head will not benefit from “UnderCover Porting” type modifications.