Using a 66 A-100 8 3/4 in a Early A body

64X2 said
From backing plate to backing plate it measures........53 3/8 inches,

Interesting, mine which was supposedly also out of an A100 is 56 1.2 inches from flange to flange. This is very close to the C body measurements.

Yours sounds like it is actually from an early B body.

Here is a great write up on the topic:

http://www.bigblockdart.com/techpages/875axle.shtml

Thanks,
OldMoparsRule!