If you have replaced the valve stem seals, then the valve guides themselves could be worn, or the seals were not fitted all the way over the guides. If you have constant smoke then you could have an engine breathing problem or worst case scenario, something wrong with your pistons. If the smoke is actually steam, then I would look in your breather and oil filler cap for a light brown sludge. The head gasket could be on its way again. Did you have the head skimmed last time? The cause of going through head gaskets could be a slightly warped head.
