I think you are mistaken. Are you sure that it was a 8 1/4 not a 7 1/4?
They were typically 7 1/4 sbp. slant 6 cars. (9" brakes) non power brakes.
I've never seen a factory 8 1/4 sbp.
If a '73 and up was ordered with power brakes, it automatically got the bbp discs and a bbp rear diff. (regardless of engine)
If a '73 and up was a small block it automatically had bbp discs and rear diff.
I personally own 3 a-bodies '73 and up with 7 1/4 bbp rear diffs.
One is a slant 6 '74 dart sport hang 10 bbp power brakes 7 1/4 bbp.
Two is a '73 scamp 318 manual disc brake bbp and a bbp 7 1/4.
Three is a '74 dart 4 door 318 manual disc brake bbp and a bbp 7 1/4.
All are automatics.
I have 5 '73 and up slant 6 a bodies manual brake cars and all are 9" drum cars with sbp 7 1/4 diffs.
I have seen many more that coincide with this option. (or lack thereof)
All cars are factory equipped and unaltered.
Do you (or anyone else) have any pics or other documentation to the contrary?
I'd like to see it and then i would eat my words.
Chrysler also didn't make a body 8 3/4 rear diffs after the '72 model year and definitely not in bbp either.
Thanks,
Tom.