I'll let you guys in on a little secret. The motor is hollow. My buddy gutted the motor and cut the housing shorter. Then cleaned it up real nice. My wipers didn't work anyhow. Car is never in the rain.
Their should be a way to make your idea actually functional. Might have to get a smaller but higher amp rated motor, but something should fit. As for making a varialble speed unit out of it all that would need to be done is a rehostat put in series with the motor and switch. A direct bolt in functional replacement would probably sell like hot cakes between the Nova, Camaro, Firebird, and Ventura guys. Might have to tear one apart and see what I can do with one of the metal fabricators at our shop.