I have a Schroth 4-point system in mine ("Rallye-4"). It uses the stock front lower mount points (lap belt) and the rear belt mount points for the shoulder belts. There's no drilling, etc.
www.schroth.com
Cost was about $190 for one seat, deluxe with pull-handles & shoulder belt pads. Write up & pics on the forum main page, as well as
http://members.home.com/fhurderjr/sho.htm
I have also seen a more custom install incorporating a bar behind the front seats, attaching to the B/C pillar area, and using 5-point Simpson harnesses.
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White 95 SHO MTX
"Jezebel"
Spends more time in the shop than in the driveway
Teases me with occasional great runs
3.2L SHO Shop Long Block
9psi Vortech S-Trim Supercharger (12psi on the way)
Eibachs, Tokicos, 96 Brakes, SFCs, STBs
PP Y-Pipe, Custom Stainless Cat-back, Bullets
http://members.home.com/fhurderjr/sho.htm
Mocha 91 SHO
"Monique"
Hi-Revs Jr. Clutch
SHO Shop Y-Pipe & Cat-Back
120k miles, NOT RUNNING YET (Grrrr!)