I can't agre with running it without a thermostat. The thermostat acts as a regulator for the coolant flow, and without one, the coolant will flow too fast through the radiator and will not transfer the the heat effectively. There needs to be some sort of restriction in the system to control the flow. At minimum, you sould at least run it with a washer in the housing, I beleive 1/2" ID would do, to restrict the flow (old Flathead trick). As far as filling it goes, there really should be no problems filing it, as the radiator sits higher than the highest part of the engine. The trick is to fill it slow and let the air escape. You should be able to clearly hear it hissing past the thermostat. Then you would want to run it to purge the air, it will do it on it's own as it heats up and the thermostat opens.
If you are concerned about traping air in the engine, you could remove the temp gauge sensor from the back of the head (highest point on the engine) and then fill. But I don't really fell that would be necessary.
I should mention, I work on cooling systems on a regular basis, and have never trapped air in the system. Even on cars with the radiator lower than the highest point in the engine, which is pretty much all late model cars.
