1966 vs 1967 Barracuda front bucket seats

The seat frames are almost identical. Bun width changed the seat width. OEM 67 buns would be small for 66 skins.
The seat back frame is made of several pieces welded together ( like everything else ). The upper member of the earlier frame was round tubing. For 67 they reformed that member to square and punched holes for head rests through it. The main difference is about 3/4 inch difference in overall height. So a 66 skin on a 67 frame may need a little extra filler here.