I mis guessed the mirror Motor size,
It's good news they are closer to 2.8 inches. That means you could use the existing mirrors and just re stick them to the trimmed down Corvette mirror mounts, that snap onto the motors..
Your remaining task would be to mold your old mirror cowl cases in plaster and do a fiberglass pour into the bee's waxed plaster molds.
Yala! New cases that you can work the mountings for both the motors and the mounting studs to the car body.
I'm sure this has got your gears turning.. Ya Gonna like the way they work tooooo..
Also use the 85 Firebird wiring schematic's and harness to help you marry the Ford Two way switch into the harness..
You can make your own harness, and get the terminals and connectors cases from Amerian Autowire. They use the common Packard flat terminals simualar to VW,, That I like, and approve of.. Use a 6 wire trailer harness kit from a auto parts store.. That will help make it easyer as well...
Been there,, Done that... I could make it for you, But NAAA! you need to learn how it work for yourself..