I can offer a little insight into valve size increases, supported by lots of dyno runs, but they're all from small block V8's. There's little if any noticeable advantage at low to midrange RPM, and it takes some pocket porting, good carburetion, and exhaust port and manifold work to get a noticeable advantage at higher RPM.

Since a stovebolt engine has all the breathing ability of a sprinter with soda straws duct taped into his nostrils, the gain from lots of valve size increase, porting, dual exhausts, and multiple carbs will be moderate at best. You can actually make a silk purse from a sow's ear, but it makes the pig look really funny!
Jerry

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