If your 66 seat frames are in good shape, you can re-upholster them and add bolstering, and even headrests if you want. 66 mustangs have the perfect mustang interior, in my opinion--the Pony interior. I'm a big enough fan that I used it in my 67. TMI's Sport R interior adds substantial bolstering to the front seats, along with alcantra inserts, but retains the classic pony look. They have optional headrests that you can have added if you are looking for the safety of a taller seat back.
Note: Not endorsing the retailer listed in the picture, this is just the best image I found in a quick search. I have no relationship with TMI, and don't get any benefit from my sales pitch.