Got mine today, looks pretty sweet! It'll fit nicely inside the 25gallon propane cylinder I plan on using as the shell of my heater. Just above this, there will be a heat exchanger coil from a hot water pressure washer to pick up the heat and send it off into the garage.
Draganof-
Use two radiators. Use a thermostat control and zone valve to direct heat into the space, and when the zone valve closes, it'll just waste the heat out of the second radiator outdoors. You want to set your heat level and thermostat temp to keep the waste radiator from costing you too much extra fuel.