There are many threads on aftermarket seat frames and the tips to installing them on a first gen suffice it to say none are easy. Many involve brazing in new bolt locations in the floor pans, or hacking or adding new seat rails.
But you have other options using the original GM seat frames. The first gen seat frames are basically the same all three years, just with or without headrests. You can buy new 69 frames at many of the "big box" on-line vendors such as Ricks and Classic Industries, or refurbish your existing frames and even add headrests if they are not included on your current frames.
On those original frames you CAN have the best of both worlds...seats that look modern, and are comfortable to sit on but that also look stock, and more to the point install as stock.
Take a look at TMI products "Sport Foams" & Seat covers for your first gen http://www.tmiproducts.com/chevy/bro...ort-seats.html
If you are looking for modern style and comfort these are the top pick . They look great, feel great and fit with no modifications to your floors and rails. The new foams convert the existing GM frame into a more comfortable ride.
The Sport Seat covers match pattern, color and style of the originals, and according to the website pop-up - soon TMI will be introducing custom seat covers for resto/mod tastes.
I upgraded my 69 Pace car with the Orange and Black Houndstooth, and there are several threads on these projects to look at here on TC as well as TMI is a sponsor.