In your el camino if you don't have headers or a larger cam then stock, you probably wouldn't even notice the thrumming or deaffening drone that gives you a headache, usually aroud 2,000 to 3,000 rpm in a cruise with a slight load on the engine(up a hill) the car will usualy try to shake itself to death then. Cross over pipe is the same as a balance pipe or even an x-pipe. Usually try to install them as close to the collector as possible but still infront of the cats or mufflers if they are in the way. The tailshaft of the tranny or the first couple of inches of the driveshaft is probably the easiest place to mount them.