Mancini Racing still selling Yukon axles that are 3/8"-1/2" too long

I have 2 Abody 8.3/4 sbp rears one is complete drum to drum with 2.76s in it that needs a rebuild(from a 71 Demon),the other is just the housing. I have the bbp brakes(drums,shoes,etc). I think im going to go with axles from Dr.diff do I just tell him what third member I have i.e. 489 or 742 case and they will know what axles I need? I can fix just about anything on my Dart but I dont know alot about rear diffs. The other question I have is about the drive shaft. Since I will be swapping an 8 3/4 rear in place of the 7 1/4 rear but retaining the 904 trans for now do I need different U-joints or a different drive shaft? I have the drive shaft from the Demon that donated one of the 8 3/4 rears and another drive shaft from a 74 Dart Sport 6/904 with an 8 1/4 rear will either of these work?