This is an old thread, but I went through this recently, replacing the wrong-year Camaro seats in my car for some reason, so I figured I’d share my experience.
Replacing the seats from scratch is a bear.
It would be best to find seat frames that are not repop’d.
Used ones are hard to find, and even old beat-up ones go for a lot of money, at least $300+ unless you’re lucky enough to find them locally; you also have to pay arm/leg to ship them.
Then you have to buy new seat foam and seat covers. IIRC, even with the cheapest foam and covers, you’re looking at least $700-$800 with shipping and tax, and decent ones even more.
The seat foam is also expensive to ship.
Then you have to pay someone to do the upholstery, probably $400-$800.
I usually do everything myself, but the upholstery is something you need a specialist for.
Also, expect at least $100 for other odds and ends for the seats.
Oh, and I don’t know about 79, but there were a lot of yearly variations in seat styles, which complicates things even further.