I would just center it on the frame. Find a couple good spots on each corner and check the overlap from the frame to the edge of the box. Also keep an eye on the distance between the box and the tires. You might also ensure it doesn't twist as you tighten it down. Another measurement could be to check the box for being square. Measure from inside corner to inside corner. The distance should be pretty close to equal.


1946 1-ton Panel
1952 1-ton Comml. W/Grain Body