hello 63 shortbed.

basically pull one of your original wheels and measure from where the wheel bolts to the drum, to the outside of the rim.

here is a link on how to do it.
http://www.customwheelsonly.com/Custom_Wheel_Backspace.htm

hope this helps.
leonard


1957 Chevy 3100 1/2-Ton
Photobucket pictures
In the Gallery