You might want to be ready to replace every piece of weather stripping around the windshield, top and the halo bar. When mine was replaced earlier this year (no, it farmed it out), after they installed the windshield piece, the old top side pieces didn't seal properly with the new front piece. After replacing the top side pieces, the rear didn't seal properly.
It seems that the old weather stripping on my car had shrunk some and the new pieces were thicker creating a mismatch between them with each addition.
This all started with the windshield being hit by a rock - getting that replaced properly and mating up with the side moldings and new weather stripping caused me a lot of grief and caused me to change companies doing the work. But that's another story.
The first guy, from a 'glass company,' installed an aftermarket weather stripping around the windshield and it didn't fit worth a damn. After 2 tries, I told him to go away and I took it to my favorite dealer. They used genuine GM weather stripping - at twice the price, but it worked. So, I'd suggest investing in the GM stuff.