Most of the reasonably intelligent racers I know plan out their build around the rules, with the chassis first, figure out how light they can afford to make it, then figure how many cubic inches that they can run at that weight.

one local guy I know just couldn't get his SBC light enough within the rules, at 275-something radial class and this was a 3 stage nitrous car running 4.8's. Now he runs a 582 BBC, and is working on making 4.5's. S/F....Ken M