I bought the rear axle from a '70 one ton because it had 4.10 gears and put it under my '57 one ton. http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/70-1-ton-back.jpg
I'm using an adapter to run dual type wheels as singles. http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1040.jpg
http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1041.jpg
The only difference between the '57 axle and the '70 axle was the length of the lug bolts (due to the '70 having single wheels) and this 1" thick spacer which I assume was necessary to allow the 16" single wheels to clear the drums. http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1146.jpg
I had to modify the adapter because it had 5/8" studs but I'm now running the original 19.5" dually type wheels. I think that with longer studs I could run my duals on this axle with no problem.

'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc