Wide cop car wheels?

I have a set of cop car wheels on my 79 model SWB truck. I would like to get the rears widened to 10" with about a 4-4.5 backspace or buy some already completed...is there somebody who does that or does someone know who sells them already done? Thanks

http://www.specialtywheel.com/

I have 4 that have been widened to 10" with about 7" backspacing - run them for 10 years with street tires and slicks. They were done by Wheelworks in Florence, NJ..