My suggestion is to read the specs for the classes you want to race in very carefully as they likely won't be the same. The specs for SCCA Solo aren't the same as those for wheel to wheel SCCA T1. Not sure about NASA.
My second suggestion is to let the pros at Phoenix do it. They take the interior out and weld in a custom SCCA T1 legal eight point cage. They are a forum sponsor with a link to the left.
A lesser cage will not work for any SCCA road racing (as in door to door) classes, though I'm not exactly sure what you mean by "lesser" classes. The C5/Z06 is classified in T1. Not sure what, if any, other classes it is legal in.
I believe that the NASA rules are close the SCCA's in regards to door bars, etc, but I'm not sure.
If you know what class you want to race in, I may be able to help more.
The C5/Z06 is only classified in the T1 class with very limited engine modifications. Internal modifications like mine are illegal in T1. Therefore I have to race in another, likely faster, class. I also am looking at the open road races like the Silver State Classic. It looks like the T1 type cage is the way to go, unless anyone has other suggestions.
Unfortunately, I don't think you'll find an SCCA class your car fits in for National racing. There is a regional class called ITE that you may be able to get it into. To the best of my knowledge, ITE is a catch all class where cars that are classified in other sanctioning bodies can be run. Thus, the class has a wide variety of cars. Ex SCCA World Challenge cars are the ones I normally see at the front. So... if you build your car to another sanctioning bodies requirements, such as a NASA class, I believe you will be able to run it in ITE.
I will say, all the ITE cars I have seen have had full cages based on the original sanctioning bodies requirements.
A roll cage is a big dollar investment, so as I mentioned above, I would make sure you get a cage that meets the rules for the class you decide to run, whatever that may be. That will take some research on your part, but will prevent you from wasting some big money.
Subdriver, thanks for the ideas. I am going to look at the NASA and SCCA rulebooks for exact requirements. I was not too worried about competing in national events with this car. If I get to that level, I will find another car to tear up.