Installing H Pipe

Want to install an H pipe on 2.5" exhaust with Hedman Hedders and pipes out the side in front of rear wheels. So the space I have to work with is limited, I know to be most effective you want to install close up to the header outlet. My question is: Is there an effective distance between the two pipes, or does it matter; can you also screw thinks up if the distance is too great?
Thanks