Don't know if you plan to shave the firewall and put the wiper motor under the dash (sorry, don't have time to go back and look), but I found out a while ago that later FJM bodies used a plate bolted to the wiper motor that would bolt up to a minivan wiper motor (little tiny thing) and bolt to the factory opening in the firewall on an A-Body.
Hope that made sense.
The mounting plate bolted to the wiper motor on the Diplomat setup that I grabbed. Just had to dig the bolts out from under the rubber gasket they glue over the hole plate to seal it to the cowl. Don't remember how the minivan wiper motor bolted to the mounting plate, but seems like it was easier to get apart than the Diplomat one.
Just a thought.