ok, the anitsubmarine belt was designed to pull a driver out of the floorboard (basically) of an open wheels car. a street-driven car doesn't place the driver in a position to slide under the dash. yes, it CAN happen, but is unlikely. the lap belts (when adjusted properly) hold a person relatively upright. the anti-submarine doesn't protect from a broken sternum. i've also had harnesses in just about every street-driven car i've had...had a bunch of cars...
anyway, the reason many harnesses won't pass DOT regulations is due to the fact that most idiots wouldn't be able to decipher how to use the belts. they have some sort of regulation where everyone in the whole world would have to be able to use them. further, they (harnesses) have to be certified evey few years where the common seatbelt doesn't.

the SPARCO bar is pretty cool. it is a good place to start for harness mounting. they also offer a little bit of chassis stiffening, but no, they are no roll bars. the SPARCO belts are made by Sabelt (TRW), so save yourself some money and buy TRW's if you want them. tehy are on some sort of national/international backorder, though. the Simpsons are fine as are RJS and Bill Simson's new line (can't remember what they're called). the other guy up there brought-up some of the other manufaturers who have harnesses available. i've also used offerings from different European manufacturers. you can choose things like upward pulls or downward pulls for the belt adjustment. you can choose camlock or 'standard' latching belts. my Jamex set had regular 'clicking' belt buckles like normal seatbelts. they're fine. and they tend to be nice for around-town driving. expect large dings, dents, and to lose big chunks of paint when using harnesses on the street. they tend to cluck down when exiting and entering. it also sucks when you passenger uses them and doesn't give a rat's ass about your car or your belts...they tend to close the doors when 1 or more blets are hanging-out...

the rear most (floor) seatblet anchors available from the factory should be ok, but it's dependent on how much angle is on the 'drop'. i have to look for the info regarding how much, but i'm sure it's out here on the web somewhere.

 

The problem is not a hold-down point, the sparco bar is fine for that. If you are in a bad enough crash, the body of the car will deform, and you will not move, right? In the case of a rollover, the rollbar will keep the roof from collapsing onto your head, as there is no room to duck out as with a three point belt. This is assuming that you are actually going to be driving on track with the harnesses - driving on street with them is just stupid. (I tried for a week with my old car, its nothing but a pain and an inconvience.)