NINJA EDIT: statements in italics below were added after Tom quoted the original post.
the rules state (i added numbers in parentheses for clarity in my interpretation below):
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Eligible Vehicles:
(1) Any four-wheeled, production-based vehicle that was originally sold as a passenger vehicle is allowed.
(2) Vehicles may either retain production frame rails or equivalent unibody structures, or they may use a tubular frame provided they retain the production body.
(3) If the vehicle uses a tubular frame, then modification of production exterior bodywork is only allowed provided the end result is substantially similar in general appearance to the original vehicle. Sweet box flares are specifically allowed.
(4) If the vehicle retains production frame rails or equivalent unibody structures, then exterior bodywork modification is unlimited, provided no safety rules are violated. Production frame rails or equivalent unibody structures may be modified to alter a vehicle’s wheelbase.
(5) Vehicles that don’t fit these requirements may ask to run for exhibition only. Vehicles running for exhibition will be marked with “Exhibition” stickers.
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SO, in the case of the Bradley GT Body (not the very rare complete production Bradley GT Vehicle), that body would only be Challenge-legal by adherence to rule (4), ie as an "unlimited modification" to the body of a vehicle with production frame rails or equivalent unibody structures.
Most Bradley GT bodies were attached to VW pans, but the Challenge rules don't restrict you to a VW pan. You could use an F-150 frame if you wanted to, alter it to the desired wheelbase (which is allowed under (4) above). in this case, your Eligible Vehicle is an F-150 with unlimited body modification. Or you could modify the Bradley GT Body as required to allow it to attach to, lets say, an 1988 Fiero "equivalent unibody structure", and in this case your Eligible Vehicle is a Fiero with unlimited body modification.
or in the case of blending a GT6 (iirc that was one of the vehicles mentioned) with a Fiero, you could unlimitedly modify the GT6 body to attach to the Fiero equivalent unibody structure, and your Eligible Vehicle is a Fiero with unlimited body modification.
or etc, or etc, or etc. just like when riding a mountain bike, don't look at the obstacles. look at the clear path between the obstacles.
IMO, YMMV, Once upon a time, call your doctor for erections lasting more than 4 hours, etc.