Did you test the thermostat in hot water before installing it?
If your engine was not warming up without a thermostat, and your engine overheats after installing the thermostat, the thermostat is a prime culprit.
Also, after testing a thermostat and assuring that it works, it is a good idea to drill a 1/8" hole in the thermostat to allow a little coolant to flow through it all times. It is possible that you do not have coolant in the head - where the temperature gauge sender is located?
