WABOOM: Try your idea outside the car and see what happens. I don't think it will hurt the starter. Fordor41 also has a good idea.
Here's a helpful thread.
https://www.jalopyjournal.com/forum/...-matic.622529/
and
https://www.fordbarn.com/forum/showthread.php?t=48011
If possible, PM folks with Mercomatic starter like yours, get their phone number and call them to discuss how they have installed this starter.
IDEA: Clamp the starter in a BIG vise using the case as clamping point.
Put on a leather glove and simultaneously twist and push in the starter drive (CW or CCW) to see if you can manually retract it.