Sorry I’ve no pictures but I fitted an old heater in my 36 Cabriolet... I mounted it on the raised centre section of the firewall. It was tight but I think it looked better centred than off to one side.I changed the inlet and outlet pipes with copper fittings from the local plumb shop... 1/2” od solder type fittings and tube . The outlet ran to one of the bottom hoses with one of the fittings that go through the wall of the rubber tube... the feed was from one of the water pump fittings and through a heater control valve operated via a cable.The original 6 v fan was seized so I obtained a 12 v motor and made it fit the heater. I could have, on reflection, used one of those flat square fan assemblies that you see on computers etc... you can get them in various sizes and they are nice and slim so would give you more room to push the heater back a little.
Just found an old picture of it... blow it up and you can just make it out.