The gills in later Panteras do not have threaded studs. They have posts that push into plastic grommets. The grommets have become hardened over the decades and gills can be difficult to extract. The gills, posts, or the paint and sheet metal could be damaged if forced. You can see if you have grommets from the back side of the wheel well. If you have them the gills will need to be pried off carefully. Maybe use a heat gun to soften the grommets.
Once the gills have been removed they will likely push on and off much more readily.
