I ran into this problem about 30 years ago. The 283 in my 57 Bel Air had an Edelbrock StreetMaster on it, with crappy looking stock Chevy PCV style valve covers. The breather, (oil separator) can under the intake was missing, because it wouldn't fit. The solution was to modify the breather so it was shorter front to back. I just used a hack saw to trim off part of the breather, so it mounted further back in the engine block's valley. Not sure how much I trimmed off.....1/8th inch??? 1/4 inch??? 1/2 inch??? It wasn't very much, though. The mounting brace that is spot welded to the breather can just needed to be bent a little bit, so it bolted up in it's stock bolt hole. Heightwise, the breather can stayed the same, it just got moved slightly further back in the engine block valley. It's worked fine ever since. Still have the same car with the same engine. The StreetMaster had some cooling fins on the underside of it. These were on the exhaust crossover. I had to grind some clearance on these, too. Did not have to get into the crossover passage in the intake, though. The intake had to be modified in a couple of other ways, too. It only had the boss for an oil filler tube, without any filler hole. Used a hole saw to drill one in it. It also had a small hole connecting the intake plenum to the exhaust crossover. I tapped this hole and put allen head pipe plug in it.