Marlo,
The Hydramatic trucks are fairly rare; they also usually came w/ 5-window cabs and the deluxe trim package: fabric seat cover w/mathcing door panels, two tone paint inside & out, stainless window trim, and other niceties.
Hydros were available in 1/2, 3/4, and 1 ton trucks: GMC in '53, Chevy in '54.
