"I did find the crossmember in the Factory Assembly Manual (section 7, sheet 3.05) for Automatic Transmission."
The reproduction "1947-1954/55" Factory Assembly Manual is actually a 1954/55st FAM.
There was no rear-transmission mount (or rear-transmission frame cross-member) in 1954. The rear-engine cross-member (and motor mounts) changed in 1954, allowing for the elimination for a rear transmission crossmember (unless you had a Hydra-Matic transmission).
It is quite common to find an old 47-53 truck with a missing rear transmission crossmember/mount (it was removed and never replaced). GM installed that cross-member and rear transmission mount because GM thought it was necessary.
