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you wouldn't have to run really low compression especially if you were going with all forged internals. so the difference from the stock block to the modular 302 would be significant. and then of course once you added the F.I. it would be even more significant.
That all depends on how high you go and if you are using aluminum heads and all that jazz. I've seen guys run 11:1 on pump gas with some good flowing aluminum heads, but that's as high as I've ever seen.