MFW,
There were, of course, many types/models of H-M transmissions. Talking about what you do not have will not help you much (even if you wanted to know the useless information that there was even a mid-50s 21-speed H-M transmission used in very large GMC trucks).
The 1954 1/2 ton GMC H-M transmission (1954) was most likely the same as the 1954 Chevrolet 3100 H-M transmission.
There were minor differences between the 1953 GMC 1/2 ton H-M transmission and the 1954 1/2 ton H-M transmission.
Note that your 1954 transmission was a 1-year Chevrolet truck transmission. Your 3100 transmission is set up for the enclosed drive-shaft. The 3/4 ton and 1-ton H-M transmissions in 1954 were open drive-shaft.
I have a web-site that shows the complexities of the 1954 3100 H-M transmission, if you are interesting in seeing the steps in a complete rebuild (let me know). You might also be interested in seeing
these 1954 Chevrolet Truck Hydra-Matic service manuals. 
