Builder Certification and rollover protection
One tank supplier I use has a manufacturers EPA certification sticker on tank and another for the door jamb w/build date. I could not get any fabricator to build a replacement aux tank for my Suburban because of liability issues.
I suspect using aftermarket racing tank and proper fittings, filler, venting to outside of cabin and grounding would be a safe bet. However, I'd also setup a rollover venting system to be sure EPA/DOT regulations are followed along your safety and the safety of others as priority.
Anyway, whatever approach you decide upon keep it safe and secure, GOOD LUCK!
I suggest the following FATS system for rollover/venting protection, I use it!
1999 GMC K2500 BURBAN, NAVISTAR enhanced 6.5 td aftercooled, dual alt's, PSC p/s pump w/HD cooler, redundant FSD's, HEATH program, turbine/downpipe wrapped, 4" SS exhaust, real time OBD2 data logging w/device controller, EVANS waterless coolant @ zero (0) psi & 135 gpm pump, 4L80e w/kevlar and premium steel w/cryo treated input/output shafts running @ 100 deg. F & 140 deg F towing, all synthetic fluids, AMSOIL bypass system, MileMarker hydro, dual fuel tanks, and on and on, questions PM me.