[Drbrown]

"IMHO" thinking: Given the three engine mounts in a '47, such as I also have, (2 pads in front; one saddle clamp pad on rear of tranny), combined with the frame to engine longitudinal anti-chatter rods .... if in good condition and properly installed, I believe there is little to no opportunity for longitudinal (front to rear) movement in the engine-tranny-torque tube assembly within the car frame considering they are all bolted together. However, as noted in this thread, the movement of the internal parts and acceleration and deacceleration of the car produce varying ROTATIONAL stresses in the assembly .... i.e. clutch pedal and transmission shift connections. The universal joint allows some minor internal front to rear movement at the torque tube shaft.

Also, lateral (side to side) shifting of the rear of the body produces lateral stresses on the rear of the torque tube and axle-wheel assembly due to the flexibility in the rear spring (the stock sway bar may reduce this), shocks and rubber tires.

The "popping out of gear" during acceleration/deceleration/inclines/declines (hills) together with the shift in car body weight suggests to me that something is happening in the shift linkage between the car body and tranny .