''I'm going to fix it up someday'' The resurrection of a '74 Dart Sport......

It's been a while since I looked in and you have covered a lot of ground since then. Nice to see it as a runner again! That has to be fun!

Ideas for your heat problem.
Replace existing thermostat with 160F unit if cold weather operation isn't a big thing.

You could block off the heat crossover in the intake manifold. They sell intake gaskets with the tin plugs or a thin piece of sheet metal/shim stock will work too. Little longer warm up required and manual choke.

Insulate the fuel line with some foam wrap like they sell for water heater pipes. There are other things that work but being automotive products they cost more. The fuel pump, being aluminum, picks up a lot of heat from the block it's bolted to. Combined with the gas we have nowadays...

They also have isolator plates that are about 2 mm thick which could be added under the resin spacer you have now. Some folks even make their own spacers out of wood!

Make sure the water pump is an AC spec unit with 8 blades as opposed to non AC with 6. Flow Kooler pumps with the disc flow even more water.

Add a little Water Wetter/Purple Ice to the coolant. Eases surface tension of the water, lowers the temps 5 to 10 degrees and keeps steam pockets from forming as the coolant passes behind the combustion chambers in the head.

Make sure the fan clutch doesn't have a bad clutch in it.
Retarded timing can cause overheating especially in a heavy, highly geared car. Just thoughts that might help!
Looking forward to more progress!
Alan