MAR-K’s bed assembly instructions explain that once the 4 pieces of the bed are assembled (bed sides, front bed panel and rear cross sill) you need to make sure your bed is square.  If it isn’t, then see the instructions below.

There are a couple things to check to get your bed square. First, be sure the bed sides at the rear are square to the rear sill and that the complete bed is sitting on a level surface.  If one side is leaning inward and the other side leaning outward a like amount, the dimensions will be same both top and bottom, but the bed will not measure square. You can check that with a carpenter square.  Sometimes it helps to install the tailgate to keep the sides vertical during the assembly. That way they can not shift very far if you have to loosen the rear sill mounting bolts and they can be adjusted back to square after the bed is on the truck.

If your bed is still out of square 1/2" or less, it should be readily shifted to square by loosening all the bolts that are holding it together and pushing the long dimension to make it smaller, and the small dimension will get larger a like amount.  It may require holding it in the square position while a helper tightens all the bolts.  If you are holding it square when you tighten the bolts, it may still move slightly when you release it. A bed is slightly flexible before it is on the frame and before the wood is installed and tightened. When you install the wood and bed strips, be sure to again hold it square while tightening all the bed strip bolts.  The truck frame will keep it square when you install it on your truck.