Those engines were designed for a very different purpose that you want it for.

If the engine has the stock pistons, closed chamber iron heads won't help much. Stock pistons are .080" or more down in the hole. And that means that there is no way to get any quench at all so detonation is very likely. I have worked with these engines quite a few times. In fact, I still have a couple in the barn.

Improvements certainly can be made, but they detonate like heck with stock pistons so it usually takes a lot of trial and error to see what they can put up with. Optimal timing is impossible to get.

Do you have a budget or level of difficulty in mind? The better flow with new aluminum heads would help and would work with any future improvements, so that would be the first thing I would do. Then cam, intake, carb, and headers. In other words, the only desirable parts on the engine are the block, rods, and crank. Even the oil pan should be changed for more capacity.

twocents