I didn't see a reference to wheel adapters in the rule book but I would be leery of using them on a heavy car at high speed. Re-machining the hubs for a new bolt pattern is a better idea. I went through the same quandary and finally had my 5x130mm Porsche rear hubs re-machined for the standard circle track standard 5x5" pattern.
Regards, Neil Tucson, AZ