It sounds like whoever rebuilt the motor may not have gotten a good seal on the head gasket. This would cause water/coolant to get into the cylinders and that would cause the white smoke/steam from the exhaust pipe. Since the coolant is leaking in from the top, it would not get down into the oil pan. I would think if this was the issue, the engine would not run particularly well, especially under load but if it's a small amount in each cylinder it may not be too noticeable. Before you rush into something, let's see what others have to offer regarding this issue. The way I see it, it just about has to be head gasket.