A lot of other threads on this subject. Just do a search with key words: Water and Heater.
... lots of info in some of them.
I am doing a hot water heater in my cabriolet and have been searching ebay regularly looking for good looking vintage heaters. Most water heaters were aftermarket in the thirties and forties and many have attractive cases. Search ebay Motors for: Hadees, Harrison, Hot Wave, Arvin, Tropic Aire, and probably more brands I can not remember now. Many are about 7" to 9" tall, about same width. Also, Snyder's has a "kit" for attaching heater hoses to the A's cooling system. Newer aftermarket 12V heater from Vintage Air is too tall for attractive firewall mount below the gas tank on a '29, may fit better on a '30-31, I don't know; but a restored old vintage heater can fit and look much more attractive.