Does yours by chance have extra steel welded onto the frame rails? I had a very similar 64 or 65, 25ish years ago, and it had 4.56 gears, poly 318, dash shifter, 8 lug wheels, and it had a short flatbed installed when I got it so not sure the original bed style.

But the frame rails had additional steel welded to the the underside to make the overall height of the frame taller/thicker. IIRC it was like a 3/4" or so flat strap the full length of each frame rail. Supposedly this was because it was a Forestry truck for the conditions in which it was intended to be used.