I'm dealing with that just the opposite (62 truck/65 bed), my mounts are off for bed to frame, the '62 frame is wider, I had extra rails so I think I'm going to just cut off the spacers on the extra rails and put them in the proper place and weld them on, my back bumper is off by several inces on each side so I will make a second set of brackets to fit to the frame as for it is a stepside wrap around bumper which is bolted to the fenders.
So yeah be sure to measure up before you have to take it back off.