IIRC, the A-pillar and top weatherstrip retainer are held together with a riveted piece where they meet as
slow 442 mentioned. Meaning the Upper section and front section is essentially one big channel piece that flexes right there in the corner where the blow out retainer piece sits.
You'll need to remove the weatherstripping on the A-pillar and remove the screws on the A-pillar retainer section too. Then that whole piece SHOULD come off.
I THINK. It's been a long long time since I played with any of that stuff. Someone who's done this recently on a HT swoop in and save my azz.
GM p/n 20737664 and 20737665 for that entire piece for HT. T-top cars use 2 pieces on the A-pillar. The channel on the A-pillar, and there's a top corner piece with the blow out retainer.