[shortbed70]

Brace the frame in front of where you will be cutting and level it front to back and side to side then when you place your main crossmember you have a solid level way to square from. Once you get it tacked in tack your two rear hoops on and the back crossmember and square corner to corner... If everything is square ad level your good to go. You could also pre assemble the entire rear structure and set into place but I would advise against fully welding until you have it in and it can't move. I am a elder by trade so I've built a lot of structural frames for projects and never had a failure yet... Good luck and don't hesitate to ask questions.