Sqarin' a Bed

[BMERDOC]

I added a 3/4" plywood floor to replace the completely missing wood floor. I went to add the tail gate on only to notice a big gap at the top between the bed and tail gate. I measured the bed and found that it was an inch outta square. I was able to move the passenger side forward to gain 1/4" Now the drivers side needs to come back as close to 3/4" as possible. Trying to assemble the bed with the plywood was kinda like trying to build a piece of IKEA furniture without directions. How did you square your wood floor bed? BTW, the perimeter bolts are all removed and the plywood is bolted to the chassis.