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You might want to also check out oldGMCtrucks.com
That year GMC engine and dash/gauges are different from a Chevrolet truck.
Brakes and most front end parts would most likely be the same.
Most likely you have a 100 series - 1/2 ton?
A Maintenance Manual should be available from parts vendors.Here is an early 1955 manual with info on the H-M transmission.
You might as well drain the transmission and put in the correct fluid.
I do not recall if there is a transmission filter (members here should know).
That is a complicated transmission, if it needs repair.
The good news is that it is a good/strong transmission - it might need repairs.
Look here in the Tech Tips for advice on checking out a new truck.
In the Tech Tips, look at the 3.55 Ring & Pinion conversion (for a 3.55 ratio, instead of the 3.90 ratio).
You would need to obtain a 1948-1953 ring-gear carrier (GMC or Chevrolet 1/2 ton).