after changing the thermostat in my 1994 lincolnmark 8 it still over heats

I had the same problem last year. I had to take my '95 Mark to a radiator shop. The drained the block, filled the system with new coolant and put in a new thermostat. It has been fine ever since. I had tried to add more coolant to the reservoir but after the engine heated up the tank was empty and my temp gauge was on hot. The problem ended up being that the coolant was so low in my engine that I wasn't getting enough of the coolant into the engine causing it to overheat.... Mike