The 124 headrest swap probably won't work because 124 headrests are attached to the rear deck, not the seat. It would be major work to cut the rear deck out of the 124 and weld it in your SD.

I second getting a seatback with headrests from a wrecking yard or parts car. Any 126 without reclining rear seat is 'functionally' a donor.

Sixto
87 300D