A shot in the dark but here goes. I had a similar problem years ago with a 235. Rough idling, timing problems, etc but not the smoke.
I found a split in the steel vacuum line which runs between the distributor and the carb. Thought nothing of it but changed the line anyway and the problem was solved.
As I say a shot in the dark but who knows. Good luck.

1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice