I put a 99 D80 in my 92 D350 and it was a near bolt in. Only modifications I had to do were bore out the mounting plates for the leafs (4" tube on D80 vs ~3.5" on th e70) and shorten the driveshaft 1-1/4". Brake lines fit, and with the 13x3.5 shoes, it improved my braking a bunch.
For the better braking, bigger guts, and outboard drums (do brake job w/o R&R of the hubs), I'd live with an open diff for a while. Put a limited slip in when you slow down a little.
FWIW- the dually axles are the same width wheel mounting surface to wheel mounting surface.
I have a friend in StLouis with an extra D70, but I dont know if he wants to sell it or not. 3.54 DRW P/U, LSD
Dan