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There are several kits on the market to install headrests on 65-68 low-back Mustang seats. I suggest taking a look at them and applying the same design principles to your bench. Besides the headrests, the seat will need some interior structure to mount the headrest slides or bracket. The structure should not be allowed to pivot in case of a hard impact to the rear. Preferably, that structure is welded to the backrest's structure.
Personally, I prefer headrests that go through the top of the seat rather than mounting behind. The stem(s) poking through the fabric can be hidden by escutcheons or grommets.
I recall a FTE member here who retrofitted some to his bench seat. Use the advanced search feature and hunt down the thread. I think he only spent like $30.