[Mickeymotormount]

Alot of times the cab settles over the years due to rotting rubber cab mounts or mushy cab supports or a combination of both.

I believe the bed should just bolt on. I would think the brackets on the frame are the same for fleetside or stepside beds.

Perhaps when the new cab supports were installed, the cab was not jacked up to account for the settling that occurs over the years which can cause the body lines to not line up. Maybe you can add some shims to the rear cab supports to help with aligning the body lines.