I have a Vintage Heater in my 64 Signet, made by Vintage Air and I could not be happier. Its small, has 3 speeds and 2 defroster ducts. The unit is easily installed under the dash and the heater duct opens and closes automatically. This heater works fantastic and I hardly ever need it on high. Check out the size of it.
http://www.summitracing.com/parts/vta-506101