[clay68c10]

You can try shifting the body around, it may work.

The frame could also be out of square, not uncommon after 40+ years of being a truck.

Find 4 equal points on the frame and measure them cross ways (x) and see if the measurements are the same.
For example, I used the 4 farthest away from each other bed bolt holes on mine. It's 3/4" off. Not surprised since my bed doesn't quite square up with the cab. The truck has been wrecked, abused by me years ago as a teen, and was a Wyoming ranch beater before I bought it.