Any of the factory 350 heads are excellent for this goal. None of them is better than the other in stock form regarding flow. The only variable is compression ratio. To get the best all around 9 to 1 ratio use a set of 68 or 69 2bbls heads. #17 and 47.

The 18s are the highest compression stock heads but nothing special otherwise, not even screw in studs.

300hp at the engine is a slam dunk with stock parts and a set of headers. I'd shoot higher. opt for a good roller cam matched to the heads, factory Qjet and manifold, have the machine shopplung cut the bowls on the head for the best bang for the buck flow improvements. Really that easy.