The Factory Assembly Manual shows the various cross members
The "serviceable" part I gave above is the cross member at the rear of the 1/2 ton's Hydra-Matic transmission.
The "cross member" under the 54 rear motor mounts is called an engine rear support (Sect. 2, Sheet 9), part number 3703340, 7.229 (same part on 3100 & 3600). I'd bet that these were bolted for 1/2 ton H-M trucks and riveted for standard transmission trucks (I'll not bet on the 3600 H-M trucks).
