Here's the '70 axle. The part that I'm calling a spacer is part of the hub. http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1146.jpg
Here's the '57 axle. http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1147.jpg
They're identical other than this spacer and the length of the studs. I think the spacer was necessary to run 16" single wheels.
The adapter is to bring the wheel out so that dually wheels can be used as singles and give the correct track width. The adapter is actually intended to be used on the front hubs to convert single wheel trucks to duals. You can google wheel adapters and find them in several places. Here's one: http://wheeladapter.com/dually_adapters.php
I got these because I was not able to find 19.5" single wheels and I plan to put a 9' stepside bed on this truck eventually. I think it will look okay with the dually wheels as long as the track width in the back is close to the same as the front. As I stated before, the adapters came with 5/8" studs so you'll either have to drill out your wheels or change the studs.

Last edited by jfnar; 12/20/2012 11:29 AM.
'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc