Why does my 350 engine runs rough and smokes(.

The smoke is probably due to bad oil seals, they didn'thave much of a seal as it was an o ring, there are better ones available called positive seals. They cna be changed without removing the cylinderhead if you have air pressure available and a valve spring compressor for on the car. Most mechanic shops can do it. They weren't the best champs for mileage either. Have it checked by a pro for maybe time for a carb overhaul or if a tbi unit check for a blown gasket beneathe the unit, spray carb cleaner at base front and back if rpm changes it's leaking vacuum