i dont think your gonna find many people on these forums that have done what you are talking of. i would recommend getting an aftermarket exhaust from a company that has experience in the field and has proven results. but if your hell bent on gettin some "custom" exhaust that your not sure is gonna work to the potential of other aftermarket exhausts then go to a local no-name exhaust shop and tell them your plan and ask what it would take.
PS: if you do decide to go aftermarket, UUC, Remus, Dinan, Stromung, etc. are good name brands.
<img src='http://img268.echo.cx/img268/1430/zkwsig23iq.jpg' border='0' alt='user posted image' />
1996 328is Black
Suspension: MB Motoring 18x8, Kuhmo Supra 225/35/18ZR
Audio:Eclipse CD3414 HU
Lighting: ZKW W/ DDE PLUS