1987 Nissan Truck 1987 Nissan Truck overheating

Engine Cooling problem
1987 Nissan Truck 4 cyl Two Wheel Drive Manual

Truck starts fine but when it warms up the heater blows warm air as usual but then stops blowing warm air and nothing but cold air comes up while the temperature gauge on the dash starts rising a lot. I hadn't noticed any problems until it overheated the other day. The rad took a lot of water to refill. NO visible signs of leakage when parked. Have replace the thermostat. This morning (after replacing the thermostat) the heater worked until a couple blocks from home then it stopped blowing warm air again. Also, when the truck was started this morning, there was a huge cloud of white smoke from the exhaust. This has never happened before. I have read the help books on this and nothing is jumping out at me. Also, the water pump was replaced last year. There are no noticeable noises either while idling or driving (low speeds or high speeds). I am baffled