The data plate breaks down as follows: G=Gas, C= cushion (solid tire), 52= model (manufactured late 50's- early 60's), 20=2,000lb capacity 24= 24 inch load center, UF=fluid coupling- if memory serves me right

L1ftmech is correct- washing an old unit with cleaner & letting it sit is a sure fire way to get the clutch disc to stick. getting the clutch hot & hoping for the best is the only way I know of to get it to stick. In extreme circumstances I've had to "break" the clutch loose by chaining the unit to an immovable object & letting the unit pull against the chain with the clutch depressed.

This may not work in your situation because I believe your truck came with both a clutch & a fluid coupler. There may be a sticker stating "equiped with fluid power" or you may have to pull the floorboard & check the bellhousing thickness.