Is the rear feed chain loose, and banging on the door hardIJ You might try some foam weather stripping that is used under truck canopy's that mount on the truck bed.
The door needs to be held up against side angle irons and the old pin lockup system wasn't the best. Take a look at the system they used on the '84 or newer machines and maybe you can dupicate that on your older machine without buying the floors. Good luck.