2001 Dodge Ram no heat

hi, recently i took my engine apart because it was having problems with the lifters. after that, i put everything back but this is where i made a mistake of not replacing the head gasket. now when i start the truck, there's cloud of constant white smoke coming out of the tail pipe. am i right on the head gasket part ? is that what is causing it to act this way with the smoke ? also, my truck now has no heat. obviously it was working before i took the engine apart, but after that now there's no heat warm up the truck and the first time the temperature gauge went up way too fast. i thought maybe there's a problem with the thermostat, since i noticed it was rusted but di not bother to change it. so one day i took out the thermostat and put in one that i had laying aroound, which was working before. so i started the truck, i can feel a little warm air coming out, but as soon as i switch the position of the vent to other position. you know, like changing it to the windshield to the floor area. it started blowing ice cold air again. what's going on ? i even flushed out the two hose that goes in the heater core and it seems free. there's no coolant smell or leak inside the cabin. please help !!! thanks to all