Don't know if the design is the same or not so this might not be valid. I had a 67 mustang with the same problem. I would actually bend my linkage when I pressed the pedal. It turned out there is a small bracket screwed to the inside of the bell housing that the throw out fork would pivot against when the pedal was pushed. this had broken and the fork was pivoting against the bell housing. This shifted the pivot point away from the throw out bearing by about 1/2 inch. That was enough change in the geometry to drastically increase the force needed to move the bearing.