my my trucks with SM420 the transmission has no support, only front engine mount and one either side of bell housing.

The transmission with provisions for a rear mount likely came from an application with two side engine mounts, and the back of transmission was the third, no front engine mount.
Or with a torque tube rear a rear transmission mount may be necessary?

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-