Nothing for now. CG-Lock and stock belts
Long Term, the plan that's coming together in my head goes something like this:
1) Swap my reclinable pseudo-racing seats that are on sliders for fixed FIA buckets on solid mounts (bolts to adjust, like the normal ones you see with vertical stripes of bolt holes), which have proper holes for anti-sub straps.
2) *Then* put in a well-designed 4-point rollbar, with the harness bar set at the right height based on where my shoulders end up in the new seats, pad it with something that makes a little more acceptable street-wise but is still SFI-rated (dual-density or orange-aid or something like that), and install 6-point harnesses at the same time. Leave seatbelts in the car for street usage, and be extra careful.
3) When the car gets to the "rides to the track on a trailer" stage, upgrade to a full cage and rip out the seatbelts and the airbag system (well, and lots of other changes, but those are the ones most relevant to everything here).