I have 5 main caps taken from a 312.  Lined them up on a table today and noticed one had bolt holes that were at least 1/4 in. shorter than the other 4.  Numbers on 4 are ECZ 6329A, and on the oddball ECZ6325A.  The short cap came from the rear of block.  Dont recall reading that there were 2 styles of 312 caps.  Did the '57 312s have the shorter bolts?

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.