Tweaked card cage

I've got a PDP8/A chassis which has a card cage that has been tweaked off in the front just a hair. Enough that you can't get the cards in by an interference amount of clearance. Anyone have any idea on how they handled this? I'm thinking of using some blocking and then a 36 or 48" pipe clamp to apply diagonal persuasion to get it back in true. The rear of the box is not impacted. In the body shop trade this is called "diamond" and usually gets your car totaled if your frame has that damage term applied to the chassis. I don't see any other damage other than this, no chip damage or cracking to the back plane, which is of course brittle. The rear dimension of the chassis is good, and I can't see what caused this. Could have been a drop, or could have been under too many layers of crap and was tweaked. Other clever ways to apply very gentle but forceful pressure to true up the cabinet is appreciated. Also how much to go past square to account for the "spring" back of the steel once the force is removed would be nice too. Thanks Jim