Chassis/Cage Tech question

[BR3W CITY]

So this has had me curious since a while back when someone posted up a snippet from the NHRA rule book about cage specs. I don't need a full cage, but I'm approaching rollbar territory and it bothers me:

The rule was about position of the hoop and cross bar (belt bar?). It said that the bar had to be X number of inches above and Y number of inches behind the drivers helmet. (I don't have the current rule in front of me, I won't pay for the book )

Thats where the issue comes in. My cab is so tight, that it would be physically impossible to mount the seat in a way that could have enough space to fit a crossbar in the cab, as well as the hoop...and still have it "behind" my helmet.

What I need to know is.....is it OK to build 70's-style EXTERNAL main hoops? Its totally possible for me to get the 4-6" of room I'd need, but putting the hoop from the frame in between the cab and the bed...and the "lip" on the back edge of the 66 roof would tuck a bar nicely.

The best answers I've had so far are "well if it fits the rules"....but I don't wanna put that kinda time and work into something that will fail tech the second I show up.