I'm not sure I'm going at this the way you're asking...My understanding is that the 312 engine and 368 engine distributors can be interchanged.
To do this, you have to change the distributor drive gear.
The 312 uses a 14 tooth drive gear while the 368 has a 13 tooth drive gear.
I've been told this interchange has been utilized by those desiring to use the '57 Merc Turnpike Cruiser tach drive distributor in the 312 y-block. Apparently, the Turnpike Cruiser tach drive distributor is virtually identical to the '57 Ford T'Bird tach drive distributor except for the suffix on the distributor housing part stamping number.
IF you want specifics on how identical the centrifugal advance springs, distributor cam, and vacuum advance diaphragm part numbers are, I need to connect with an acquaintance who has a Mercury parts book for '56-'58.
Hope this helps.
