I did all mine in the last 2-4 yrs I paid a low of $40.00 to a high of $120.00 for a set of seats. They are out there you just have to be patient & look every day...I used craigslist & found them all within 40 miles. Any year will work just make sure the seat is in fair condition & you can always add more foam to the seat if needed. I reversed my seats so the inside that was never worn is now the outside. You can make your own brackets from directions on Binnie's site but you have to be dead on with the bend on the bracket & drilling & tapping the holes. He doesn't charge that mush & supplies everything including the metric bolts you will need. Mr  Mike supplies a great video & everything fits perfect. I supplied my own material on the 77 as I wanted to keep it a close to original brown (it has brown & yellow seats) as possible  on the others I used his material. I also had a different logo sewn in on each car.

I did put C5 Seats in my C3.  I would not pay $250 for the kit pieces Mid America is selling.  I'm very non-handy and I tapped a couple of holes in the C5 Seats to match them up with an extra set of C3 tracks I bought online.

I have also heard late model Mustang (I know that is a vulgar word to some Corvette people) seats also fit. I'm sure the tracks would need to be modified.

I used a set of 79 seats in my 74. Made a big improvement in looks and comfort from the stock seats. 

just got done with c5 seat in my sons 73