That's a very good solution IMO. I can definitely see myself doing this. I'd love to get rid of that ugly motor sticking out and getting in the way of everything.
I haven't scoped out an A-body to see how much different things would fit. From the measurements that we took it seemed impractical to use the 2-speed motor that protrudes further away from the firewall. It was about 2" longer. The additional length would have required us to space the motor further inside and caused more issues with wiper arm alignment. Our Charger originally had a 2 speed but we'll convert the wiring to the 3 speed when we re-wire the car. They bolt up the same on a B-body. I'm not sure on an A.
Moving the wiper motor behind the firewall was going to interfere with the heater box too. It didn't look like we'd have had to do much more than notch one of it's corners but we haven't tried to fit the enclosure back in the car after moving the motor. I'd assume that something similar would need to be done on any other car. Our Charger was originally air conditioned so the size of our heater enclosure may have been larger than a car without.
I'm the welder so I'm the one that needs to apologize for the quality of the welds. Any remnants of POR-15 really screw with welding and even with the dash out I had to make overhead welds while lying on my back.