Toyota 4Runner 1995 blown head gasket

1995 4runner, 2wd, 220,000 miles. I bought it 8 months ago with 200,000 on it from a donation yard where you pick out the car you want and the yard fixes whatever is wrong with it. My 4runner had the engine replaced with a used one that had 67,000 milesl because the old one had overheated. A couple of months ago it overheated but I never let it get to close to the red mark on the gauge. So, I took it to a shop where the themostat and housing were replaced along with a couple of hoses that were leaking. They then put my car back together and pressure checked again. At that point they told me their was a hose that comes off my water pump that was leaking and that it needed to be replaced. I then took it to the dealership where they replaced my water pump timing belt. After about driving it for 4,000 miles without overheating again, white smoke kept appearing from my tail pipe when I started it. Then again it started to overheat. Now I believe I have blown a head gasket. My question is: The dealership said they pressure checked the car for any leaking(including the head gasket) after replacing the water pump, and was obviously ok because they released the car to me. The car didn't overheat again until after the head gasket blew. Would'nt my car have to overheat again before the head gasket blew? Does this seem right to you? Also after I got the car back from the dealership my heater didn't work anymore. Please help explain what could have happened. Thanks[/b]