The guy was tellin me also that most Chevy v8's take that size and the is how he figured it. Is there a formula that u can use let's say from the bore n stroke of the motor

Not sure if a header was ever built for the L series...but 2.5 cat back can be bent local. Not picking on L cars but you'd be better off with a 2006-2007 ION with the 2.4L ecotec, or any year ION Redline.