I was watching a YouTube video today about the build of an off topic British car, and they mentioned a company in the UK called T7 Design. Among other things, they have a line of compact self contained heaters in several different size. They look like they could be adaptable to many of the cars built here on the HAMB: https://www.t7design.co.uk/heating.html I have no affiliation, just thought it may be a useful source for someone. I know the topic of "what heater can I use in my car" comes up from time to time.
Those electric heaters don't usually put out much heat but draw amps like crazy. The 5KW midi water heater at 170 British pounds = 201.28 is a little higher than the small Vintage air heat only unit or the Old air Heat only unit.
Several small decent US built water type heaters are available, so I wouldn't order from an overseas supplier.
I started with a defunct SW Southwind heater and built this. It has a Ford truck heater core. The blower will be up behind the dash and out of sight.