Mmkay, so I borrowed my brother in law's digital caliper, and here are the findings.
I have a bunch of windshield wedges I kept for people looking to do a resto mod project on an older/classic car. This is pretty much the standard wedge you'd find on the vast majority of cars from roughly the late 70s on up. Case in point, the Fiero guys love upgrading to these mirrors and they fit directly on their factory mirror wedges.
It looks like it's coming out to a 100th or so smaller than the one in your car, but pretty damn close. They have a ridge on the side facing the inside of the car which is a tad wider then the rest of the wedge. That ridge slides into a grove on the mounting area of the mirror. That grove is a hair wider than your wedge, so I think it should fit fine. But it's still best to find a 'normal' mirror out of another more modern car and try it on the Eagle to make sure it fits. You'll also notice that even on the right pairing of mirror to wedge mount, the wedge doesn't go all the way up to the top of the mirror mount (there's a small gap at the top). This is not an issue as it's still on there snugly, and once you tighten the torx screw, it's not budging. Pics are below for everything I mentioned.