[dsraven]

nice work.
when I did a frame like that I used a pre cut tool to drop in between the frame rails and then used a ratchet strap to keep the rails at the same width as I built cross members etc. also, because I work alone, I used a length of light chain, like dog leash chain, attached to a pin on the front of the frame and then stretched to a predetermined spot on the opposite rear corner, just to check the corner to corner dimensions to ensure the frame was staying square after welding things onto it. the pins on the front were removed after. a simple rivet nut in the frame at the same place on both sides so a bolt can be screwed in when needed and when done the rivet nut can be left there ot removed easily. I have also used cross bracing before that is easily ground off after a few cross members have been installed to form a ladder look.
curious what the rear suspension is from. nice fab work there.