A-2.5.5.4. CHASSIS/BODY/INTERIOR
(c) Roll Bars: Roll bars are permitted. Full interior roll
cages are not permitted.
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When do I need a roll bar or roll cage, and what should it look like?
A roll bar is required in any convertible running 13.49 seconds or quicker in the quarter mile, and in other cars beginning at 11.49. The roll bar is accepted in vehicles running as quick as 10.00 second e.t., provided the stock firewall and floorboard is intact, other than for installation of wheel tubs. The rollbar must be constructed of minimum 1 ѕ inch o.d. x .118 inch wall mild steel tubing, or 1 ѕ x .083 chrome moly tubing, and must conform to the following diagram:
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So you don't need a roll bar till 11.49 et and you don't need a roll cage till you run 10.99 et. You cant combined metric nut with standard bolt, because that's what you trying to do. So forget the idea for roll cage and just get a harness bar and you should be ok.
By the time you get to 10s you will know what to do. If you ever get to have an autocross/10s car let us know.