Ralf : Just to make it clear, the Retractors you pictured were "Standard" on all Models except Convertibles. The Deluxe Seat Belt Option included "Push Button Seat Belt Buckle Release" (as opposed to "fingertip" release, and the Seat Belt Warning Light. Retractors were not part of the Deluxe Seat Belt Option.
In the 1960-68 MPC, under "Front Seat Belt Assembly", for 1967 Mustang Convertibles, it specifically states (for both Standard and Deluxe Seat Belts), "Without Retractor". In addition, at the bottom of the listing it notes "For Retractors, See Accessory Section, Group 611A06", which shows the "Accessory Retractors I previously pictured.
To use the ones that were Standard for Coupes and Fastbacks would not only require major modification to the Rocker Panel, but could be an issue for your safety.
Unfortunately, those of us with Convertibles will have to settle for the "Accessory Roll-Ups".
Bob