If it smokes after a period at idle (max engine vacuum) I might go look at the valve stem
oil seals???
Do you get a puff of smoke when you re-apply the throttle after being on the over-run? If so, that would tend to confirm my valve stem
oil seal idea.
You would have to have very worn bores and rings to see the smoke as the effect of excessive
oil slung out of the crank IMHO.
I'll come right out and say I don't know enough about sbc engine architecture to know if an intake leak can give you excessive oil into the exhaust port. Just working from general principles here.
HTH
Wilf