I too needed new seat belts in my 124 Spider. I found it was cheaper to have an old set "re-webbed" by a local canvas awning shop. My 72' had only front seat lap belts. (not much protection) And nothing inthe rear for the occasional rear passenger.

I salvaged a nice set of X1/9 "female" buckles with clean logo's on them. They age much better than in open cars. Then I got the "male" buckle from an early-mid 70's Audi. (Opels work too) It's a low profile buckle that I was after that mated to the X1/9 side. (Most 70's German cars used "Robbins Mfg." or "Klippan/Autoflung" as their safety belt supplier. Fiat used "Robbins" so look for any German car with robbins belts. They are pretty common in the scrap yards. Anyway, they bolt right into the existing holes under your back seat. (Remember to get them re-webbed 1st.)

For the fronts, I used another set of X1/9 buckles mated to the retractor AND shoulder harness from a 72' Opel. If you have your original FIAT owners manual, they depict the "optional shoulder harness". Ever notice those threaded holes on top of your rear wheel wells, under those plastic caps? thats where you bolt the shoulder harnes from the Opel to.  Hard to describe, but picture the shoulder belt fixed in position to your rear wheel well. It runs over your shoulder where the other end has a flat piece of metal with a sort of "button" welded onto it. That "button" clips into a notch cut into the "male" tang thats on the end of your retractable lap belt. Shoulder slack is taken up by a loop and slider just like a regular old "non-retractable" lap belt. (Like the ones I just described for your back seat.)

If your over 35 then you already know what I'm talking about. Most 70-74 American had similar arangements. Anyway, I think this set up works best because it's added protection, looks like it belongs in the car, and is "new" if you get it all re-webbed. Cheaper than buying new, and you dont have to cut up your back seat to fit the 80's Spider seat belts. -SF