B-body 8 3/4 axle for A-body car!
The following "and other great info" can be found at:
http://www.moparts.org/Tech/Archive/axle/21.html
Section 3: Part Numbers and Dimensions of Axles and Housings.
Body line Track Perches Notes
------------ ---------- ----------- -----------------------
A-body, 65-72 55.6 43.0 4" lug bolt circle,
Offered in heavy-duty apps.
B-body, 62-70 59.2 44.0
B-body, 71-72 62.0 47.3
B-body, 71-72 63.4 47.3 station wagon
C-body, 64-72 63.4 47.3
D-body, 64-72 63.4 47.3
E-body, 70-74 60.7 46.0
Also check this one:
http://www.moparts.org/Tech/Archive/axle/16.html