3.91 Sure Grip Rear

What year model did it come from, I have a 1974 duster, the length from drum to drum is more narrow on 74 and 76 than 70 thru 73, even though the perches is factory 43 inch, if someone pulled this from a b body 74 or 76, it will be about 58 to 60 inches ,where my 74 a body is 55 inches, and if its an 8 1/4 from a 87 thru 90 dakota 2wd, the perches is on bottom where the 4x4 was on top those years, and most a bodies came with the lowest gear as 355 ,unless it was a special order