I have an 84 Z28 305 and the upper rad hose and top half of the rad is hot but the bottom half of the rad is cold and the lower rad hose is cold.The temp guage goes up to the red zone.I changed the stat and bled the system but it still overheats. It worked fine when I parked it in the spring and now when I want to use it for a winter beater I have this problem.Water pump shot or blocked rad any suggestions? Car has a mechanical fan.

 

What is the time between starting and gauge going into the red? I would first verify the gauge reading by using a test gauge just to be sure you aren't chasing a ghost.

Assuming the new thermostat is installed and operating correctly, the next thing may be a blockage or air in the system then the pump. A trip to the radiator repair shop may not hurt, have them flush and check for blockages. You may even flush the whole system while you are at it. A more distant possibility is a head gasket, combustion gas leaks into the cooling jacket can cause overheating but you would probably notice some coolant overflow and or bubbles in the coolant if this were the case.