this will amuse many, but my old, feeble mind seems to have misplaced the answer.
so, are the [what i would call "upper"] block mounted part of the mount assembly the same [interchangeable] between C and B bodies ? this is for the biscuit type mount, NOT the spool type. TIA. [i don't know what i would do without you guys refreshing my memory ! ]

Yes, they are interchangeable.

My GTX was a bondoed up empty shell when I got it, I pulled my first 440 out of a '73 new yorker and (eventually) bolted it into the car using the C body brackets and mounts. It's a good thing they worked, I didn't have anything else to use at the time.

greatly appreciated gents ! i "thought" they interchanged, but was not 100% sure.
you guys are the greatest ! immense thanks !