What was done to the heads before they were put on? Did they knurl the guides, or replace them? I have seen some engines smoke under high vacuum because it was pulling oil around the valve guide. Next time you pull spark plugs, do a leak down test. Should tell you if you are loosing compression by the rings.
I rebuilt a Pontiac engine that after about 20,000 miles started smoking a little on start up and again after coasting down from speed. I had asked for bad guides to be replaced, found out later they had knurled the guides instead.
I'm with Hotrod Lincoln about the rings, but this may give you something else to look at if the leak down test seems okay.
