then again,if a b body 8-3/4 fits our m bodies then a b body 8-1/4 will too.
yes,no?
Yes, the '68-70 8¼” will fit FMJ's.
Even the '69-70 7¼” will fit, as well – but not sure why someone would.
The '68-70 8¼” - sure.
The '66-70 8¾” is hard to go wrong with unless you have a High Output engine – which then a Dana 60 might be the answer (which '66-70 B-body D-60 is also a direct bolt in).
One important note is the propeller shaft lengths are different between all of the above mentioned differentials (bigger ring gear = shorter propeller shaft).
The only M's that came with 8¼” are: most (but not all) police cars.
Most (but not all) station wagons (same applies to F wagons).
Cars with a trailer towing package.
and lastly, special order cars.
Some 4-speed cars came with 8¼”.
I have also seen cases where factory ran out of 7¼” on assembly line and used 8¼” instead – but this is rare.
I would make an educated guess that 1 out of every 25, M's came with an 8¼” - but that is only a guess (the number might actually be smaller - but not higher).
BudW