I am not sure of exact numbers but would say, maybe, 1 of 8 FMJ's came with 8-1/4" and other 7 of 8 came with 7-1/4"
Most (but not all) station wagons and most (but not all) police/taxi's came with 8-1/4".
The '66-70 Mopar B-body differentials fit with little work (7-1/4", 8-1/4", 8-3/4" and 9-3/4").
The Jeep liberty's will fit with some work (cutting off and welding on external tabs to convert from rear coil spring to rear leaf spring suspension. Another plus is most (but not all) Liberty's come with rear disk brakes (with also require some modifications to work) - but the Liberty rear disk brakes is "mostly" a bolt on change for any 7-1/4" or 8-1/4" diffs.
Some have used Ford 8.8" differentials - but I have no experience with them.
A person can use pretty much anything - if you want to get axle tube cut/re-welded and axles cut/re-splined. The D150 pickup differentials are about eight to ten inches wider than FMJ's.
They are pricy, but indestructible, Dana 60 (9-3/4") are still being built - to your exact specifications (ready to bolt in) - but expect to pay out the nose for one. There are a couple of forum members here that have 'em.
If you have a pickup, a good upgrade for it would be the 9-1/4" or 9-3/4".
BudW