There are a few companies that make custom rims for this purpose. I'll see if I can come up with the info for them. My laptop with my archives, crashed.
But, on my 99 dually (different brakes) I used a Weld Alumnastar 2. 0 rim and had a custom center made for it with a 8 lug pattern.
Just get the width you need for your tire and then get the max backspacing (or pretty close to it). They offer a from 12-15 inch wide rims with up to 7" of backspacing, without oredering custom. What tire are you wanting to run? Size etc...
The alumnastar has a bolt in center section. So basically unbolt the center section. Have a reputable machine show copy the dimensional specs. Then cut a piece of aluminum with the same outer dimensions, just with a different bolt pattern.
Here is a pic of an alumnistar as they come out of the box. I'll see if I can come up with a pic of the modified ones for my dually. I sold the truck 3 years ago and the rims went too.
~Wes~