If original a 1941-47 four speed would be a non synchro transmission. Sm420 started in 1948.
I think the mount he is looking for bolts to the rear of the transmission and the crossmember.
I would think GMC and Chevrolet would use the same mount.
Attachments
7447A8A6-77C3-4D8F-B89A-DF58AA75F60F.jpeg (47.79 KB, 152 downloads)
From the Filling Station website


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