The later model main bearings (midyear 1955-63) have locator tabs at the parting line between the block and the cap, instead of a dowel pin built into the upper bearing at the center. It's about a 15 minute job with a Dremel tool to put the divots into the block at the proper position to use the later bearing in the early (1954-midyear 55) block. All the other dimensions are identical.
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!