318 Running Rough / Backfiring

OK, that PCV should be decent IF you have and idle vacuum in the 10-11" or better range. The issue over the PCV is if the increase from the PCV's idle to cruise air flow settings occurs right around the idle vacuum level, then the PCV airflow is going to fluctuate up and down and mess with you idle and dump extra air in the part of the plenum where your PCV port is. That is in the front for your carb, and you are lean in the back, so that does not seem to be the issue now. I'd put that on the bottom of the list of possible issues.

I don't know Edelbrocks but there is 'something' that controls how closed the bottom secondary plates are at idle. (This is not the top secondary air-flow plate but the standard throttle plates on the bottom.) The top plate does not really close off the secondaries all that well so this bottom throttle plate closure setting controls how much air 'leakage' is introduced through the secondaries at idle and light throttle conditions. If too far open, it can be a source of excess air into the back area of the plenum. Holley actually has a stop screw that you adjust by flipping the carb upside down (or using an offset screwdriver) to adjust this.

I would also be checking the secondary air plate settings at the top. Make sure they are at the factory settings to start. There are online videos for that.

All the above is to try to find any source of extra air into the back of the plenum or intake manifold that is making the rear cylinders lean. And of course it could be carb gasket leaks or even intake manifold gasket leaks in the back on both sides. Seeing that the #7 plug is the one that is really lean (moreso than #8), I would be checking that back corner for intake manifold leaks.