I would not recommend buying seat belts from a salvage yard. If you think about it, why are most cars in the salvage yard in the first place? (hint-wrecked). Often the seat belts are damaged during an accident.
Secondly, Ford seat belt retractors don't age well. An older seat belt assembly is more likely to not retract as it should.
Just me, but knowing what I know now (from 1st hand experience), I would spend the extra $$ and buy new seat belts. That way you will get seat belt retractors that will actually retract. In addition, the piece of mind knowing that the seat belts will lock should the worst happen.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.