Thanks - helpful ideas. Taking time on this.
The rails measure and look symmetrical - a few small dents, but no visible curves.
Checking with a carpenter's square is great idea - it shows my rear cross-member is out of square with the rear ends of both rails; consistent with the short and long measurements of the "diamond".
The front cross-members arent supposed to be square, since the rails converge. Measuring those angles would take some head-scratching, so will stick with measuring diagonal distances.
Appreciate the caveat about checking for potato chip twist. Doesnt seem to be any yet.
Could the frame joints just be racked at the rivets? If so, it seems that pulling in the horizontal plane might pop them back in line without bending anything else?
