A lot of the difference in horsepower numbers before and after 72 was the change in the way engines were rated. Prior to 72, horsepower numbers were "gross" and could be optimized, much like you see on a dyno in a YouTube video. They went for the best case scenario, and then the marketing department adjusted the numbers to suit. Beginning in 72 numbers were "net" and were to better represent the "as-installed" performance. So the engine had to have the production air cleaner and exhaust system installed, a fan belt, timing set to sticker spec, standard density altitude, etc... The later numbers were perhaps more "honest" but even then needed to be taken with somewhere between a grain of salt and the whole shaker. A high compression 340 rebuilt to factory specs usually dynoed around 300 "gross" horsepower, and NHRA factored them accordingly. Since you'll be replacing the pistons, use the earlier style (or better modern equivalent) and you'll gain some of that back. The 72 and later exhaust manifolds were probably the biggest obstacle other than retarding the timing for a cleaner idle.