Thye should fit just fine. As far as I know, the seat mounting for all the early years of the Mustang were the same. Come to think of it, I saw a '67 fastback with Mach 1 seats at the Mustangs Unlimited show in June.
All the 65-73 Mustang seats are a direct bolt in, and all will interchange. All 74-Present seats for all Fords have the seat track width identical to the early Mustangs and will bolt in using your existing tracks and drilling one hole per track.