I know of no difference between a passenger car 312 crank and a crank from a '56 or '57 'Bird 312, Paul. I've never heard anyone talk of any differences between them either.
The dimensions of the snouts for the 292 and 312 cranks are the same for whatever vibration damper you're using, too.
As far as machining the 312 crank to fit in the 292 block, the diameter of the mains, rear oil seal surface, and the rear oil slinger need to be changed.
Click the link below for the dimensional diagram that shows the 292 and 312 crankshaft dimensions from JM's web site.
http://ford-y-block.com/crankshaftid.htmHope this helps. :}
Regards,
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York