Louie, adding headrests is pretty straightforward, but not cheap. I bought the headrest supports, the chrome bezels and headrest assemblies from Zip. The headrests themselves are all stitched up, ready to go. When you get the supports, you can see they really only can go on one way.
There is no way you can weld the supports in without taking all the seat material off. After I took the leather covers and foam off mine, I painted the frames and took them to the Chevy dealer who MIG welded the supports to the frame, took a half hour.
After that, I bought new Al Knoch vinyl seat covers, which was correct for my car and using the old foam, had them upholstered on by a pro. I guess the important item is to be sure the welder knows exactly where he is supposed to weld the supports to the frame. The four weld locations are shown in the '67 AIM.
When a '67 was ordered from the factory with optional headrests and leather seats, the headrests were still vinyl, they never had leather headrest from the factory. Good luck, Patrick :D