6x4 heads from 75 thru 79 (came on Pontiac 350s and the W72 400s depending on the year) would have a slightly smaller combustion chamber thus slightly increasing your static compression ratio but your only talking like a 5cc difference. Honestly if that motor is already 230 horsepower stock your already 10 hp over a stock W72 400 and maybe ahead of the game.
I guess a lot depends on what you want out of it. I put a set of 6x4 heads on my 400 with an aftermarket cam. I had the heads milled to increase compression ratio along with port matching the intake runners. Between that, dipping, checking for cracks, and all the other labor I could’ve had some nice aluminum heads for a few hundred more.
