Hi Mark -
Many of the GTDs competing in Sprints/Hillclimbs do not have cages that would suffice for circuit racing. The regs were different some time back and it was possible to meet the reqirements with some simple rear stays that triangulated from the internal rollover hoop, to the rear side sponson chassis rails. (hope that made sense)
Current MSA regs have changed somewhat and for circuit racing, I'm fairly positive you require a diagonal. It is all detailed in the MSA Blue Book.
Not sure how far along you are with this but I can heartily recommend the services of Andy Robinson ARRC Roll cages
- A very knowledgeable, friendly, helpful chap who produces top notch work...