I did run a full twin stainless from off the front pipes(2x3").It scraped around the rear diff area so welded small plates where it scraped. The car was too low i admit however.With only small rear mufflers,it had the typical gtr scream but with a gurgle added to it. Now have a full titanium 90mm and it is just as loud and scraped.Lesson:if you have a small gap between the top of the tyre and the guard,it generally doesnt scrape.
P.S.full stainless twin and front pipes for sale(under 300km old)In the process of putting in a couple of hotdog style mufflers to quieten it down.Unbelievable sound for track work!!