RHS heads flow exceptionally well. They are cast iron though (nothing wrong with that I just prefer aluminum, so I went eddy closed chamber heads for my 340. Part of the reason was I stuffed a hyd roller cam in the block), I believe member OU812 (pretty sure that's right) can sell them to you and port them.
X2 thats Brian @IMM engines, OU812, he built a RHS headed stroker, it was in Car Craft mag a few months ago, he will be happy to sell you ready to bolt on heads.