Yeah, separating the rear vent window alloy frames from the glass may be problematical.
I've never done it. Or know anybody who has. (Free free to chime in if you have....)
I don't know if they supplied the rear vent window as glass only back when they were spare parts, or supplied it as a complete unit ready to fit in a car - all I know is I never delivered one when I worked for a dealer. It was more likely than not sold as a complete unit.
Going to a 91+ front windscreen seal eliminates the problems there. Separating the rear windscreen trim from the rubber seal is easy; getting it back in? Well that's another story, but Redwood Chair knows the secret.