Loose the weight restriction's all together in the index classes, and just have "no tube chassis".
That way guys with unique setups that run frames (rangers, corvette's, s10's ect...) still have a category that is based closer on the vehicles ability, then just to throw them under the bus and make them all run top diesel against dedicated drag setups.
I know of a model A rat rod, with a 6bt. It has a full frame, it is maybe 2800lbs, but I doubt it would run much quicker then 11.90's and the owner has no interest in building a cage for it, or making it any quicker. However I am sure he will come out to at least some local events with it. I'm sure he would have no problems running beside Gord's Smokin' Gun @ 15,000lbs? In fact it would probably be a decent index race. To me the whole point of index, is so you can "run what ya brung" as long as it meets safety rules for that class.