Hope you haven't cut your material. The inside measurement of my 54-3100, which I just took is:
If you are going to drop the material into the 1" recess the overall size would be 50 wide X 77-5/8" deep.
If you want the total length from the front bed wall to the tailgate it's 50 X 78-1/2".
In other words....the bed wood sits down in a recess that ends 7/8" shy of the inner surface of the tailgate.
If you cut a sheet of metal and laid it on top of the 50 X 78-1/2" you'd have to add a spacer to the cross members that support the bed floor. Laying material down inside the recess would result in you having a drop down at 7/8" in front of the tailgate but your metal would rest directly on the bed cross members.
Which ever way you go, be sure to measure the bed diagonally to insure that you have the bed corners at a 90 degree angle. Failure to square the box can give you fits when you try to get it bolted down and have your tires/wheels centered in the wheel wells. Hope this helps.
Dave
