oldbluetruck,
What year(s) are you referring to regarding 3-speed and 4-speed manual transmission rear-mounts?
The frame cross-member at the rear of the transmissions was commonly removed from 1947-1953 trucks - for more convenient removal/installation of transmissions (especially the heavy 4-speed transmission). This is probably not a good thing, if the transmission rear mount served a good purpose?
From 1947-1953, ALL 1/2 ton Chevrolet trucks (and, I think all 47-53 GMC 1/2 ton trucks) came with a cross member at the rear of the transmission (and had a mount for the rear of the transmission). This was the case for 3-speed and 4-speed manual transmissions (and for the 4-speed Hydra-Matic transmission in a 1953 GMC 1/2 ton long-bed and short-bed).
I do not know about transmission rear-mounts for 3/4 ton and larger trucks.
In 1954, the 1/2 ton frame cross-member for the rear MOTOR-mounts changed. And, the clutch housing and rear MOTOR-mounts changed. This stronger mounting-arrangement from 1954 onwards resulted in no need for a manual-transmission rear mount (or for a frame cross-member for the TRANSMISSION rear-mount) - this was the case for both 3-speed and 4-speed manual transmissions (on the 1/2 ton - I do not know about the 3/4 ton).
In 1954, the new Hydra-Matic transmission (on a Chevrolet truck) was used with a frame that had two different cross-members than was used on other 1/2 ton 1954 frames: (1) the rear MOTOR-mount cross-member was different (different from earlier years and different for other 1954 1/2 frames); and, (2) a frame cross-member and a TRANSMISSION-mount were at the rear of the massive Hydra-Matic transmission.
