Might want to do a compression test like I said earlier.
If you can still contact the previous owner and find out exactly what he did when it was torn down. Valve job/seals or not. That might help.
Might be able to get away with valve seals IDK. Compression numbers would tell you alot more. Also if you pull the spark plugs see if you have an oily build up on them, they should be a nice tan color. Black and oily indicates oil burner.
Make sure you have the proper oil and it's at the proper level. Over full or too thin could make it a smoker too.
Love my box stock V-6