Need advice on squareing up my bed

[master 1969 c10]

My old bed had alot of body work that I didn't want to do so what I did over a year is bought all aftermarket everthing. What is the best way to go about trying to make sure I get it somewhere close to be square. Where I am at now is 90% of the factory holes do line up but when I get the tape measure out and measure coss the box it is 1 inch out. When I measure side to side front and rear it measure the same. We measure both fenders and they were both the same length I still have to put on my tailgate.So when I get the gate on. What have u guys found is the best way to go about this is trying to square this thing up is 100% to much to ask for all new aftermarket parts this is not going to be a show truck just my play around and drive it. Thanks for your input Doug