quick question, my buddies 97 z28 bone stock, he wants a cut-out after the cats what size its that exhaust tubing 2 3/4 od?? better question, what size cut out is needed?

 

The stock exhaust is an odd size, but most people go with a 3" cutout. An exhaust shop can get it in there with no problem.

 

2.5" is a little cheaper and not more than a stock piped car needs. Have it positioned right before the pipe goes over the axle. That makes it easy to get to and away from stuff that doesnt need hot exhaust blowing at it.