Are you running molly rings? I had a Boss 351 motor that did exactly the same thing for about 2000 Miles. Smoke from one side one time then the other sides at other times. The rings would not set very quick because of the type of rings the builder used. After about 2000 miles it stopped. Ran like a scalded dog but it was not fouling plugs.
If it is running fine other than the smoke then I would put at least 1500 to 2000 miles on it before tearing it apart. Make sure you do not just run at one speed all the time. You need to put the motor under different loads accelerating and decelerating.
What type of
oil are you using? Break it in with straight 30 weight and do not use synthetics until it has at least 3000 miles on it.