I played around with this problem for a while. I tried a manifold heater, but just couldn't get enough air to flow through it to warm the car. I then looked at restored water heaters, and mocked up a cardboard box of the heater sizes posted in ads, but there just wasn't a decent place to put it in an A.
Then I purchased unit made by Vintage Air that I thought I could mount high up on the inside of the firewall, but that didn't work because the A/C evaporator was in the way.
I ended up mounting it under the seat, air blowing forward. I had to cut an opening in the seat riser for a heat outlet, but it isn't seen. Once I got water flow issues worked out, and a thermostat on the engine, the heat output is tremendous. Exactly perfect for my needs.
I also knew that using inside air in really cold weather would fog up the windows from the occupants breath, no matter how warm it is inside. SO, I left the manifold heater in place to bring in just enough dry outside air to eliminate that problem.
I noticed that there were cold drafts all over (Town Sedan) I fully weatherstripped the 4 doors, sealed real good around the pedals & steering column and the lower cowl areas that are open to the engine compartment. WOW, snug as a bug in a rug!