500 would be fine then. According to the GMPP website, the crate 383 can make 457 at 3400 carb'd. I thought that higher numbers would be easier with a 408. Not trying to skimp out on anything, just thought it would be interesting from an engineering stand point to pull it off with 87. 93 it is then.
If the 408 bore is .030 more than the 6.0, what can be gained/lost from boring it .040 with the same 4" stroke.


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