Mike,
I have Schroth ASM belts in my BRZ track car. They "bolt in" and do NOT require a submarine belt. The "ASM" is their "anti-submarine" design. It would be an easier install in a street car and negate the need for the sub belt. See more here:
https://www.schrothracing.com/item/r...ning-Harnesses
In my NC Miata track car, I am running Safecraft belts. The setup I have is designed to be used with a HANS (narrower belts over shoulders), different design for lap belts, etc. When I ordered them from Charles Espenlaub at Safecraft, he said "PULL DOWN" is the way to go. One trick for getting the laps belts tighter (if that is the concern) and your seat is on sliders... move the seat back a little, tighten the belts, then slight the seat / yourself forward into the belts. They will be plenty tight.
https://www.safecraftracing.com/