According to the Standard Catalog of Chevrolet Trucks, the '54/'55 1st 1/2 tons had 3.90 gears, '53 and earlier AD's had 4.11's. There should be an ID code stamped on either the 3rd member or axle tube (depending on year/axle type) that can be de-coded to tell you the ratio. Cars & Parts has an exellent id guide that has the numbers for '46-'72 trucks, it's the Catalog of Chevy ID Numbers 1946-1972. Very informative.
Re-read your post. On a '65 1/2 ton the standard ratio was 3.73, with 3.07 and 4.11 optional. The codes are; 3.73-HA,HC,HE,HF,HG,HQ;3.07-HB;4.11-HD,HH. Hope this helps you.
