Do not add another rear mount to the transmission. 55 through 72 trucks all used the four mount system, two front or side mounts
and two mounts on the bellhousing regardless of the transmission used and they work fine.
I just recently did an engine swap in a 55 3100.
The rear mounts were mush, had the wrong bolts, and didn't have the internal support sleeves.
Yours may have a similar problem.
Fred
