The oil breather tube may be plugged,,,take it off and clean it out,,,also,,,the intake manifolds on those old 283's gather a lot of carbon and oil deposits in the intake manifold,,,take it off and clean and scrape those deposits out of the intake runners.
If your bowing alot of carbon out of the tail pipe,,it might be an idea to look at your mixture settings,,,or choke valve setting,,,couple of good runs should take care of most of the carbon build up,,,but you gotta think,,,,,the carbon had to come from somewhere right??
Lastly,,,,if that 283 has a lot of miles on it,,,then do an oil change,,and use straight 40wt oil in her,,,,,if the mileage is really high,,and you dont want to re-build yet,,,do not use engine flush,,,because your motor would have a lot of build up which is taking up the wear.
Engine flushing an old motor ( very high miles)will only loosen her up,,,and then you'll have blue smoke and knocks everywhere.