One set is to clear the pop up 10.5:1 pistons of the early 340's, the chamber is opened up to clear. The other is for the later low compression piston 340's. The pistons do not protrude past the deck.
Paul
why are there two different sets of edelbrock heads 1 reg set for small blocks and one set for early high compression 340's. i would check piston to valve clearance before i started milling heads for a 340.
