My 1989 D150 with a 318 overheated on me last night. I noticed first that I didnt have hot air coming out with the heat turned on. Thinking that was odd and knowing what that could mean I pulled to the side of the road and sure enough had steam from under the hood. I check everuthing and the top hose was hard as a rock. I let it cool down , made it home and replaced the thermostat, thinking this is the obvious solution, a stuck thermostat. Replaced the thermostat this morning in 14degree weather and I still have the same issue. I need answers quickly because this is my daily ride to work .. Could it be a water pump.?