look into early econoline rear axle stuff

The 6.75 is pretty weak if you're going to be doing any kind of aggressive driving with power. Probably better to look for a rear from a V6 pinto wagon. I forget what size it is - 8" or 8.5" someone here will correct me but that will be a bolt on and gives you room to work with more power. -BruceF

i experienced great longevity and no problems to speak of in terms of durability with the 6.75 in my 71.
 
it took the abuse of street/strip drag racing & a bit of autocross/roadracing over a 'career' lasting about 60 thousand miles.
 
the ratio was 3:55. an 'open' rear end (no posi) and the ONLY thing that ever went wrong was a u-joint / driveshaft failure that bent the pinion shaft at about 50 thousand miles during a street racing contest.
 
the rear end was behind a boosted 2.0 that was not babied at all.
 
that said i was always on the lookout for a lower gear but (in those days) i didn't want the weight of a big 9" to take advantage of available ratios. not to mention the added expense of wheels/tires and what not.
 
i drove it hard but i respected it and it lasted a long time