You might try these guys. Some years ago they put 55 ford centers in 7" rims for me, 4" backspace to mount a set of 10" cheater slicks. At that time they were also pretty knowledgeable about what centers or rims would let you mount the original hubcaps, or whatever you're using. Still worth a phone call.
http://www.thewheelsmith.net
We've also got a couple local guys who do wheel repair on new fancy wheels when you hit a curb and bend them. The one near me does a lot of dealer work on trade ins. But he can also relocate the center in the rim for the correct backspace. He's all set up to check the run out and do it right.
How much backspace do you need?