In Need Of Some 8 3/4 Technical Info

last Friday I went to Texas acres and got a 8 3/4 rear that Don assured me would fit in the duster. He said it was only 1/4" different. I did not have my note book or a measuring tape so I took his word for it.
Needless to say he was wrong! He did say it was out of a B body and was clearly marked '65 Fury 4 Dr. It has a good 741 pumpkin and a set of 2.93 gears. I would prefer 3.23. I now am faced with cutting down the tubes. Here is where I need the info.
My chart says that a B body of that year should be 55" flange to flange, the one I have is 56.5" flange to flange. I'm quite shure it has the small bolt pattern axles, which is of no consequence if I cut the tubes down to length.
According to my chart the assembly is out of an E body. Idont know if that matters.
The end result I am after is to put a set of disk brakes on the back, I remember the mention of 'green' bearings and have no idea if that enters into the mix.
the inner axle seal measures 2.69" in dia and has a part #2404215
The question I have is this a doable goal with what I now possess?
If so what are the steps necessary to get there.
Andrew