Buy a plumb bob.
http://www.nextag.com/Empire-Level-Steel-Plumb-698713882/prices-html
Rig it with 6-8 ft of string.
Remove the wheels/tires and chock the diff up on blocks so the whole mess won't move.
Drop the plumb bob in front of or behind the differential housing with the string right up against the housing. Do this in the exact same place left to right. Move as far outboard to drop the plumb bob as you can. Make your marks on the floor where the bob touches the floor. Have a buddy hold another piece of string at one of the marks on the floor, while you extend the other end of the string forward to about where you think you want the ball hitch. With your buddy holding his end of the string on the mark you made at the rear diff, tie a pen or pencil to your end of the string and move the pen through an arc, about 2-3 ft long. Keep the string tight as you do this. Now, have your buddy move his end of the string over to the other mark you made at the rear diff and hold the string tight at that mark. You pull the string taut and move it through and arc , intersecting the first arc you made. There the arcs coincide is the exact center of the whole shebang. Construct your rails so that the center of the ball hitch sits right over the mark where the arcs come together and you'll be golden.

 

dam tech , you out of air ?
just go buy the rivets on frame, there the same both sides .
cut notch same on each side top and bottom using a square after you mark top .same measurement from the same rivet from spring hanger ot bed mount hole.if the notch not right over each other , your frame rail wil be high on one side low on the other . this is the only major problem you'll have when you bend them rails together .
cut frame in two , same thing , both sides off rivets of cab mount .
bend till they meet .weld after tacking all good enough , it will pull straight .
got three of them myself .
your not narrowing the thing .
now i'm out of air ,my way .
but i like techs way too .just time consuming .

 

when you bend the frame rails together at the front end to attach a hitch ball tongue does it really matter how perfectly centered you get bend the rails together?

It needs to be pretty close to perfect so it won't track. The easy way is to run a piece of square tubing or pipe down the middle for your centerline then bend and weld the the frame legs to it. It also gives you more clearance in the rear of your truck for turning and backing up. Something like this to give you the idea, then cut the pipe to length: