Not only do the aftermarket ones fit poorly, but a lot of prospective restorers end up garnering some degree of confusion over which cars came with which sweeps, and so they end up getting the wrong ones. Maybe a sticky should be made for it? I'll try to break it down.
The kind of window sweep your Firebird or Trans Am came with depends on what options it has. There were three kinds made:
Fully-blacked out sweeps with no chrome bead: These sweeps were only available on cars with the B85 chrome decor package, which implemented a SEPARATE very thick chrome applique along the top of the doors, a small piece to meet them on the top of the fenders, and across the entire hood. The reason the window sweeps were blacked out was because an integrated chrome bead was not necessary given the larger separate chrome applique that would come standard with the package they were used on. The B85 package was optional on all other trim levels sans Esprit, with which it was standard (and yes, that includes the Lady Birds).
Thin, round chrome bead window sweeps: These sweeps feature a small, round chrome bead around 1/8-1/10 thick and were only seen on Y cars (LEs, SEs, TATAs, Pace Cars, etc.) cars.
Flat chrome bead window sweeps: These sweeps were standard on any Firebird, Formula, or non-SE Trans Am model. They feature a flat chrome bead that is a little under a quarter inch in width. This is the kind that you'll see the most often.
If I'm inaccurate on anything, by all means let me know.