The color of the ID plate would ID the size (if it is a 1954 H-M transmission):
At this link are GMC tag colors for different sizes of 1954 Hydra-Matic transmission.
Chevrolet 1954 H-M tag colors are the same as GMC tag colors on 1/2 (3100 or 100 series), 3/4 (3600 or 150 series), and 1 ton (3800 or 250 series) transmissions.
If it has an open driveshaft, and if it is a 1954 GMC Hydra-Matic transmission, it would have to be from a 3/4 ton or larger truck.
