I had a friend's motor doing the same thing. After checking it, I found the machine shop didn't use the umbrella style seals on the valve stems. They used the stock Chevy ones and the valve guides weren't as tight as they should be. Since it was in a T-bucket, and easy to pull the heads, we put the umbrella seals on and tried it. It stopped the constant smoking completely. You can change the seals without pulling the heads if that happens to be your problem. This doesn't cure loose guides however.