New engine is blowing smoke out of exhaust

So I just had a new 3.4 motor put in my 99. I blew the head gasket and warped the head and it was blowing a lot of smoke out of the exhaust. The truck sat there for about a year until I finally got it fixed. Mechanic is saying oil pressure and level is good as well as coolant, and my spark plugs are good too. He hasn’t driven the truck but has run it long enough for the rings to seat. Any ideas on why it could still be smoking even though the motor seems to be running perfectly fine