I got mine from Professionally Engineered parts (
http://www.peparts.com/). I believe our fine host also sells them. The newer seats have a track spacing that is about 2" narrower than the older seats.
I put 77 seats in my 73. The plates included some welded in ears for the lower seat belt attachments as well. You will need to do something as the Seatbelt in the early cars are part of the seat not the car.