Man, that previous owner is such a hack... 🙂
J/K.
Anyway, those retainers are usually spot welded in place. They were actually spot welded from the factory without any holes. The replacement piece appears it has holes for spot welding or screws, but I'd still spot weld and seal the pi$$ out of it if'n it were me. Rust repair ain't no joke in that area. You need to check the metal panel underneath as well. It looks to me there's some rust damage there causing an additional headache. If there was one area that GM fell down on bigtime on these cars is the rust prevention, or lack thereof, they did in the T-top area. SUCKS DONKEY BALLS.