On my truck, I don't have the hose that goes to the radiator from the driver's side head and I have a hard ass time trying to get my thermostat to open as well. I literally have to fill the engine up thru that little ass hole, I just haven't had time to take my radiator in to get a fitting welded on.
Is your stat stuck? Is it even opening? Is the bottom radiator hose getting hot up by the thermostat, check that and if it's not, your stat isn't opening?

talk to you soon,

allen