E30 Zone

Hi guys, my car is nearing completion, my m-tec 2 kit is going to be finished off shortly leading me to start thinking ahead to the new back box i'll be needing. My car is a 318i with only enough space for a single tail pipe in the back steel which will eventually be covered by the rear bumper, of course i plan on opening this to give the required clearance for a twin pipe back box. Ok, here's the question, presuming ive opened the space up enough are there any other considerations i should make? I plan on taking the car into the shop and having a back box fitted but want to do all i can to make sure the car can take the new exhaust with little or no messing around. Cheers