thermostat makes it overheat?

[GreenMystChevy]

okay, i put a 350 in my truck from a friend but before I threw her in, I put on a new set of heads. We throw the motor in, start her up and everything is fine, until we watch the temp rise until it is just about hitting the other side. Shut that thing off, took out the thermostat and she runs cool as can be.

All great until now, when it gets cold in the morning going to work. I buy a new thermostat, put it in, same thing. Just heats up. All of the hoses get nice and hot except for the top one, the one with the thermostat. What gives? I need this thing to warm up, especially for the girlfriend.

Thanks for the help guys.