Fire in the hole, smoke in the pipe

Two comments.

[1] Have you checked for/if plugs for oiling up? Maybe a clue as to which cyl are the culprit.

[2] About 40 yrs ago, I painstakingly brazed up the pushrod indent on a set of 906 BB heads. Idea was grind out the prod pinch, which I was able to do. After about 3m of driving, started blowing smoke. Brazing had cracked. All I had was a pair of smog 452 heads. Did a quick clean up, de-burr of sharp edges & on they went. And guess what? Performance was better everywhere. Lesson learned. Unless airflow requirements are extremely high, the prod pinch is not a restriction & removing it just reduces port velocity. D. Vizard now covers this in his porting books.