Seems to me the aftermarket used a 318 engine to mock up their tooling, since there were a lot more 318's than340's. The smaller 318 runners didn't hit the valve covers where the bigger 340 runners would. Almost every aftermarket valve cover bolted onto a 340 needs the lip on the cover ground down.
I'd buy that theory if the same issue didn't apply to the B/RB covers as well. Cracked a brand new set of MP cast valve covers tightening them down on a 413 back int he 80's. That's how I learned about it.
