I'm doing something similar, using small block 400 main bearings inside the original mains, with the crankshaft ground to fit. That makes the main journals the same diameter all the way back, and the 400 mains are MUCH less expensive, and more available.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!