White Smoke on Startup

I just rebuilt my 2011 Silverado 4.8 LS engine. We removed the entire block and took it to a machine shop where it was rebuilt along with the heads being redone. The truck started up on its first crank after being installed with no smoke but now after I leave it sitting over an hour or for the first start in the morning it plumes a cloud of white smoke. Coolant levels have remained the same and its been about a week of this happening. Does anyone have any explanation or way to fix this?