[BRENT in 10-uh-C]

Josh, the problem is more about flow than anything. The hot water is not being pushed enough through the core of the heater since it is probably more of a restriction than just dumping it into the upper radiator tank. You will initially gain more flow if you install a higher temperature thermostat only because the stat unit will stay closed longer until it reaches the 180 or 190 degrees and then you will be back to the same problem because as soon as it reaches the opening temperature, the forced backpressure will go away and the water will find the least resistant path.

You did not mention if the vehicle was 6 volt or 12, but do a search for a pump like a Shureflo or Dedenbear electric pump that can actually push the water with a better impeller into the heater core.

Also, at 40 mph, you are moving quite a bit of air past the body which will draw the heat away from the vehicle. While a 'non-living thing' cannot feel "wind chill", think of it in the same manner. The faster the passing wind, the colder it feels. Look for ways to insulate the interior of the body to preserve what heat you do have inside. Heaters back in the day were not as efficent as what we might feel like they should be by today's standards!!

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