Thermostat replacement?

I have an 88 K5 Blazer with a 5.7 liter. I replaced all hoses, radiater, water pump, thermostat and housing. Also replaced the heater core. The truck never overheated before but was leaking every where. After I replaced all this it began overheating. When the thermostat is in it only gets hot on the P/S of the radiator. The lower hose, heater core supply and return line, and the thermostat housing all get hot along with the P/S tank on radiator. The upper hose and D/S of tank stay cool. Some one give me some thing before set the dang truck on fire.