My '36 Ford coupe has a '39 Ford hot water heater, it was in the car when I bought it in 1952. The heater fits perfectly on the passenger side of the firewall, plenty of foot room.
The heater has a provision for defrosters, with a slide valve on the upper right side of the unit.
I know the 39 hot water heater is not correct for a show quality car, however, it is much more efficient and easier to install.
I have attached a couple of pix of the heater in my car.