I had a 73 barracuda with a 318 and a auto. put a better cam shaft in it and it ran much better(along with a set of header.) The problem was that when you revved it up it would pore out a haze of oil.
The rings, and in particular the piston ring grooves, were very likely well worn at that point, and revving with the better cam put more twisting action on the rings in the worn ring grooves. The end result here may or may not be similar.
