Well if both frame rails are Exactly the same length, your idea may work. If they are not exactly the same length, you will have to factor that in. Only has to move 1/2", each rail....theoretically. I would put a rack load on it, and using wood for protection, hit it hard all around with a BFH to "vibrate" the rivets. Especially bang the ends of frame rails toward where you want them to "move". Keep very accurate measurements from start to while banging/tightening. Probably have some spring back so go past 1" a little. To DG's comment, keep checking level/straightness of members carefully all along the process. Maybe heating the frame will help, not too hot, just 200 deg MAX...or maybe just heat cross members? Heating too long or too much will affect temper of frame, not good. I don't know if it's "bent" somewhere, I am assuming some of it was as delivered and some movement over time. If it was hit or crashed, the process may be only possible by a pro. I assume you don't have any springs or axles on it. Did you measure before removing all that and after? Are you sure 1" is all that bad if body was mounted OK, etc.? Don't rush into this until you see other similar frames are much better and you trust your measuring.
