got my 400 running it was blowing lots of white smoke, and there is some coolant in the oil. i figured it was a crack or a head gasket we tried the coolant system pressure test and a cylinder leak-down test, both of them didn't show any signs of a crack or blown head gasket. does anyone have any ideas because I am completely lost at the moment.

How long has the engine been out of service? Try changing the oil. Could be condensation from sitting too long in a damp environment.

An ounce of UV light sensitive radiator coolant dye poured into the radiator will wind up at the tail pipe or the oil eventually if you have an internal leak somehow. A UV light and goggles kit will then see the dye.

Rick