My 68 GTS came them originally "stock", but the PO had taken all seat belts out during resto, so I had none.
I bought a set of reproductions on ebay and finally got them in this week.
"I added them to my 69 drop top. I had it down to the shell and welded in plates and nuts into the side panels. I scored a complete set of front seat belts from a 74 Duster. The shoulder belts do not retract though, you can unclip them from the lap belt if you need to".
@CUDA69- Where did you put the shoulder belt inside the convertible top. I can't picture how you accomplished this?
Just my .02