Frame rails at the front of the truck are the same width from 60-66. The frame tips (horns) are different from 60-62 and 63-66, which is why the bumpers won't interchange. But inner and outer fenders, hoods, and core supports all interchange 60-66.
The rear frame rails are wider (farther apart) for 60-62, and closer together for 63-66.
Front crossmembers could have a tab at the front depending which engine was ordered and whether that engine had front or side mounts (some engines had both).
