Thank you all for your words of wisdom.
Just to clear up a couple of question marks, the motor is a permanent magnet type, the voltage is indeed 12V in the 1957 Chrysler, and the 8 amp draw is only instantaneous on startup and immediately drops to 4 amps when it reaches speed (less than one second elapsed time). I've had the motor all apart to resurface the armature and cosmetically restore it. All appearances inside are fine and the motor is very free. I do have a service manual for the car, but with this sort of thing they don't go much beyond telling you to replace the motor and see if everything works fine then.
I plan to mount the PWM in an underdash location obscure to all but the greatest scrutiny, and run the controller wires down to the evaporator unit from there.
This is an extremely rare unit of which I have have only become aware of one other in our whole Chrysler 300 Club. So getting parts or a replacement unit would be next to impossible. Therefore, if no harm is likely to be done by disconnecting the low speed wire and introducing a PWM on the high speed wire to the motor, I look at the PWM solution as being not only a shortcut to getting around the problem, but also simultaneously giving me more comfortable choices of fan speed. But, of course, the caveat here is that a PWM has to be able to work on a brush-type motor for this solution to work - and if I read your messages accurately, you all concur that it should. True?