I'm not sure where the confusion is, it's pretty simple.
The SCCA GCR governs how cars are to be built. Section 9.3.39. "ROLL CAGE/ROLL BAR" states:
"Shall comply with Section 9.4., Driver Protection Structures."
Section 9.4 has two different areas* describing roll cages:
- 9.4 ROLL CAGES FOR GT AND PRODUCTION BASED CARS
and
- 9.4.5. ROLL CAGES FOR FORMULA CARS AND SPORTS RACING
CARS
Super Touring cars are not formula or sports racers; they are based on production cars. Thus you will follow those regs. If there are any Super Touring-specific deviations within those rules, then you'll follow them.
GA
* Yes, there's Appendices I and J, but those are both for cars that have already been tech'd and received their logbooks. Since you're doing a new build, you must follow 9.4.