I see you said "you shimmed the bumper, is the actual stop screw still in place on the other side? Usually that screw and the bumper can be adjusted/shimmed to correct the seatback tilt. I have seen seatbacks broken in the area of the transition from the full upper to the the side frames. You may be able to pull the back off and look in the upper sections to check for this, but it usually won't make the seatback lean back, if cracked in this area. Really sounds like the stop adjustment, or the support section for the stop and bumper are cracked/slightly crushed down from 40+ years.