Not sure about the 5th gen seats fitting a 1st gen. However, a 3rd or 4th gen seat will fit fine with very little effort. The holes pretty much line up and if you have a power seat, running wiring is not hard at all. Also, you can find them relatively inexpensive at the junkyard (at least my junkyard). I bought several pairs of seats from a 4th gen Camaro and Trans Am for my 69 Camaro project. Every time I bought a pair, something nicer would show up the following week. With the 4th gen seats you get the late model comfort as well as recline and power seats and possibly lumbar support and heating if you want to add those features too. What's really nice is if you decide to go with a 4th gen seat, Year One makes seat covers for this seat in houndstooth to match and give that 69 interior look.
If there are no junkyards near you or you can't find any decent 4th gen seats, I have (4) pairs of 4th gen seats of different colors/styles, such as standard cloth and leather, dark grey leather from a 97 SS, Black standard leather, etc. These seats are in decent condition with the foam intact and little or not tears, but all of them bought with the intention of recovering them. I can send you photos if you like. PM if you are interested in a pair as I would be happy to send them to you for what I paid (about $60 for the the pair) + shipping.
Hope this helps.
Steve
sdkar@bellsouth.net