I believe the back window in the Jetfire and Cutlass is the same, both 1962 and 1963. Here are some excerpts from the ’63 factory service manual for the back window. Steele does make the back window gasket. I used one and it worked great, no leaks. Here’s a copy of the Steele instructions. All of my trim clips for the top and side trim pieces are the same size. If the trim clips are the same from ’62 to ’63, I found some from Northland Fasteners back in 2006. Here is a photo of my old clip next to the new one from Northland. I’m not sure if they’re still in business but here is their contact info.
615 Northeast 45th Terrace
Kansas City, Missouri 64166
Windshield Reveal Moulding Fastener #2628 (a.k.a. NO2628)
You should need 14 clips. If they are no longer in business I have enough clips left over from my project I’d be willing to part with.