How much cage do I really need?

[astroracer]

I am in the design stage for the roll cage for Bad Ast. I am having a hard time finding any data that tells me exactly how much cage I need to be drag strip or road course legal (if there is such a thing for an astro van...)
It will carry three passengers easily but, if I do race the van, there will only be one (or two) bodies inside. Do I really need to cage the full interior? Would a mini-cage (read that F-body size) suffice if it was built to NHRA rules?
Any help or thoughts would be very much appreciated.
Mark
Here are a couple of current pics to help the process.