Here is information on 1946 pickup beds. There was one bed size for each model Chevrolet truck until 1955-2nd series Task Force trucks. You could refer to them as short, medium, and long.
There was no 1946 1/2 ton long-bed/short-bed (as was available on later GMC trucks, and much later Chevrolet trucks).
Nonetheless, you are most likely referring to shortening a 3/4 bed to work on a 1/2 ton truck. I hope that illustration at the link above helps.
