Steve, check the shoulder harness retractor for any Ford car or pickup truck so equipped, of close to that year in your local wrecking yard. Some GMs may have it, too.
Piece-parts like that are extremely difficult to source new and vendors don't like the liability of helping you home-repair 40-year-old safety equipment. When I repair seat-belt buckles, I have to include a disclaimer: "For entertainment and show use only", thanks to eager (and hungry) lawyers.
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