The vendors carry a combo lap/shoulder belt for around $350 IIRC, the retractor goes in the rocker (a previous owner of my car had removed the shoulder portion of the belt, and I felt uncomfortable driving with just a lap belt, so I replaced the set in my '73 last year). The fit wasn't perfect, so I had to do a bit of cutting in the rocker to get the new bolt holes to line up, but it wasn't too bad and they work just fine.
Three thoughts though...
1) They're made in Taiwan, so if you're the kind of guy who swears you'll never put 'foreign' parts in your car that might concern you - of course, the car is foreign, so you might be able to get past that
2) Since the retractor is in the floor, you don't have the same play at the shoulder like you do with modern cars (i.e. you snug yourself in through the buckle and down to the floor retractor ... modern cars have a shoulder retractor that allows free movement of the shoulder belt unless there's an accident at which time it locks up - I keep mine so I'm snug to the seat back - of course, you can give yourself slack by pulling more belt from the floor and around through the buckle. It just takes a little getting used to when you switch cars.
3) If I had it to do over again, I probably would have preferred a racing harness, likely cheaper, potentially safer (depends on whether the roof is involved), but then there's the challenge of teaching passengers how to buckle up.
Here's where I got mine ...
http://precisionproformance.com/in1050.htmIf you're lucky, someone will chime in and tell us about a belt you can get from the wreckers for $20 ... and then I'll cry. Good luck, and welcome to the forums - post some pictures, your car looks great in your Avatar!
