I've had the p0420 engine code for a really long time and i want to get rid of it. i was going to cut out the cat and replace it with a different one but i want to do it right (or as close as i can) i want to replace my stock downpipe with the zzp dp with the magnaflow cat. sorry if this is a dumb question but does it bolt up to the stock header? here's the dp i was looking at Cobalt & Ion ---> ZZ Performance . i want to order it soon but i dont wanna order the wrong one. also, would i be selecting 2.5" for the stock exhaust?