I have done some rear changes with satisfying results on these 6's as they are mostly in the high 3 to low 4 ratio's.
High rpm/mph and a common complaint from many owners. The 3:59 - 3:69 are out there on some salvage models [ can't remember which at the moment]. Aside from incorrect speedo readings, all should be fine and in my opinion, an improvement.
I have done a couple of 113's with a change to the 108-280Se-4.5 3:27 ratio and
just love it. The best of both worlds.
If you check for salvage , do look at the brakes as there were both disc and drum on the different models.
Arthur