Apols for butting in - I have a MK1 and changed the lapbelts in the middle row (5 door) for three pointers from here:

http://wescoperformance.stores.yahoo.net/index.html

If you have a threaded hole in the B post already it's very easy to fit them.

I had the hole but no thread behind it so used non-retractable belts. This involved cutting a hole in the B post to slide a threaded retainer inside and riveting the retainer so the thread matches up with the hole in the post. All istructions on the wesco site. I could have got retractable belts but didn't want to cut up the inner trim panels to fit the retractor part behind them.

They also do ones for the occasional seats I believe (I left mine as lap belts) which I think will also fit the back of a SWB.

I found the beige a close match to the beige in my car and the belts to be cheaper than what was being asked for ones from breakers. Pic below.

Rgds