2003 Ford Expedition still overheating after a new engine

I learned that the truck I just purchased had a crack in the block when it kept overheating and antifreeze kept boiling out. I had the new engine put in 5 days ago and today it started to overheat again. A couple of days ago I did notice a lot of gurgling coming from the engine. Now my mechanic says I probably need a new radiator cause when he cleaned mine he hooked up a hose to his hot water heater to get flow through it and after he did it "the radiator was still heavier than it should be". I already know what you are thinking! I agree! But then I was told by my brother to go and have the coolant system chemically flushed and the radiator pressure tested. Does anyone understand this or can make sense of it? My mind and pockets are drained! Thank you!