[1969 GMC]

you can look at the truck's VIN to see if it was originally a 3/4 ton or not. the VIN can be found in 3 locations: typed onto the option sticker in the glove box, stamped onto a metal tag that is riveted in the drivers door jamb, and stamped into the frame somewhere (not sure where, on that one).

if your truck was originally a V8 3/4 ton truck, then your VIN will start like this: CE2...

if it was originally a half ton V8, it would start with: CE1...
if it was originally an inline six, replace the "E" with an "S".

of course, it's also possible that the truck's original body was too rusty to repair, so a p/o put a cab and front clip from another truck on there. and it's also possible that it has the original sheetmetal and a non-original frame, so it is impossible in a way to tell what kind of kooky things the p/o may have done.