Here's a picture of an aftermarket extension.
[IMG]http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1040.jpg[/IMG]
They're intended to put dually wheels on the front of single wheel trucks but I'm using them on the back to run dually wheels as singles.

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