Additionally, you put the qualifier in there that it smokes when you shift. I assume you mean when you lift on the throttle. Odds are this indicates the valve guides are worn. Either they weren't done at the rebuild, were knurled (which is a tempory fix), or they were done wrong (worn valves in new guides, poor quality guides, etc.). Sometimes you can get away with putting valve stem seals on. Most decent auto parts suppliers can help you with them. You can do it without tearing down the heads if you get one of those spark plug shaped adaptors that allow you to hook an airline to pressurize each cylinder so the valve won't drop when you take off the spring and retainer to install the stem seal.