8 3/4 rearend housing

How difficult is it to narrow an 8 3/4 housing from a "c" body to fit a dart using the moser end flanges and axles? I know I will have to practice up with the MIG welder, but how do I figure how much to cut off to get the flanges in the right place for the "a" body axles?
Any information on this subject will help.

Thanks!
-Steve :???: