I had a 76, the d-side was higher than the pass, after checking everthing out the body mount horn under the driver side front floor pan was kicked up, just as you described about yours, insted of fixing the b-mount horn at the factory they just added shims on all the others on the same side to compensate for this, everything lined up however the body didn't sit level with the frame.
I would guess if you look at the other body mounts on that side you will find metal shims between the rubber bushings and the body, they will have Ford stamped in them.
There are 2 ways to fix this problem:
1: Take all the body mount bolts loose on that side, jack the body up so you can remove all the shims, remove the front body mount bushing, or which ever one that is higher, now set the body back down onto the bushings except for the one that is higher, keep this one out, once the body is setting level and everything lines up you can see how much you need to cut the rubber bushing so it will set where it should.
2: Do the same procedure as #1 however do not cut the rubber bushing but remove the horn from the frame and reweld it in the location that is needed.
Both procedures are time consuming but are worth it IMO.