The reason for the the original transmission mount located under the torque tube ball was to support the significant downward force caused by the weight of the torque tube and the tension of the rear leaf springs. Neither of these forces will exist once an open drive shaft replaces the torque tube.
There is a school of thought, based in mechanical engineering physics which suggests that supporting the engine/ aluminum transmission at three points (front of engine, rear of engine, and rear of transmission) can lead to failure at the aluminum transmission/bell housing junction.
I have had a T-5 "hanging" off the bell housing of my '49 3100 for 7,000 miles without issue. I frequently pull a sizable single axle trailer, overloaded with parts-for-sale to swap meets, and I do not otherwise baby this truck.
When I first set this truck up with the T-5, I was concerned with the transmission hanging out into space behind the engine. I even rebuilt the torque tube mount and fabricated a third mount for the T-5. After researching the issue, I removed the third rear transmission mount. Sometimes mechanical physics can seem counter-intuitive.
Carl
The reason for no need for a rear transmission mount (under either manual transmission) under a 1954 torque-tube transmission was due to
a change in design in the 1954 and later frame and rear-engine/clutch-housing frame cross members (pages 49-51).
The Hydra-Matic transmission needed an additional crossmember and rear mount, but the 3-speed and much heavier S420 4-speed no longer needed a rear transmission mount/cross-member.
Many 48-53 owners probably get by with no problems running with no rear transmission cross-member, but GM thought is was required.
The car frame had no rear transmission cross-member or transmission mount, even with the Powerglide RPO-313.
