The rear transmission mount does not attach to the case, per se.

It attaches to the torque-tube housing on the rear of the SM-420 transmission.
That housing is the same for 3100s, 3600, and 3800s, as long as they had a torque-tube-like attachment.

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.