ks 911, Look to the world of dirt circle track racing. These guys regularly run 421ci SBC's at 7000 rpm's all night long.
Why not just use a complete 400 SBC? 17 more cubes than a 383, the same dimensions, and the power that goes with them
As far as the 400 being externally balanced, that just means you drill the flywheel, and vibration dapers instead of the counterweights on the crank. Just remember to drill the steam holes in the heads. Or you can just have the whole reciprocating assembly balanced.
I was the proud owner of a 406ci SBC once. Just don't wing it up to 6000RPM's, and miss 3rd gear. It makes a lot of pretty iron chunks
