It depends on what type of carburetors you have. There are two major types of bolt patterns, square bore and spread bore. Most performance carburetors and intake manifolds are square bore. This means that if you connecte the centers of the mounting holes, you will get a square. Your Performer RPM intake should be square bore, mine is. It sounds as if your Edelbrock carburetor is spread bore. This was used on most GM cars and trucks. It is the same bolt pattern used on the Rochester Quadrajet. If this is the case, you will need a spread bore to square bore adapter plate. Most auto parts stores carry a square bore to spread bore adapter for installing square bore carburetors on spread bore intakes. I don't know if this adapter plate is reversible or not, but it would be worth looking into.