Had the same problem on my 289 ( with standard size valve covers), especially when I did sprints and regularity track days. Had catch can with lines from both valve covers (both with pcv) going into it, still had blowback , then fixed the problem with running both lines into one then into catch can inlet and a vacuum line from outlet on the can to either manifold or carby vacuum. After a hard workout on the track I empty catch can which usually has about 100ml
oil.
Never had the problem when just doing normal driving on the road.