5-point
3"
Bolt in for the lap belt, either wrap around or bolt in for shoulder straps.
edit addition: ah - submarine strap. I forget about that one.
I use a
six point (jet pilot) setup (in the old Deztaz & the Gen IV car). It has a pair of straps that wrap around under your thighs, just below your butt cheeks and holds you in place, keeping your lower half from lurching forward during the dreaded sudden stop.
The lap belt should bolt in below and behind the hips - roughly at or up to about 2" or so forward of where the plane of the back of the seat and the floor meet.
Shoulder end should not drop lower than 4" below where they come over (or through) the shoulders of the seat.
DO NOT ATTACH THEM TO THE FLOOR AND THEN UP AND OVER THE SEAT - THIS CAN CAUSE YOU TO BREAK YOUR BACK!
From the floor, up over a cross bar (that's no lower than 4" than the shoulders) can be done, but why use the excess material - just attach it to the cross bar.