I have the catback system for my 93 gt in which the tips are turn downs. Soon I'll be putting on a cervini 93 cobra rear bumper and I know I need LX tips. Today I spoke with magnaflow and they stated that I could swap out tips instead of getting the complete catback system for the Lx. Once you go on their website you have to buy from vendors and since I'm a 50 resto fan I rather buy from here. My question is which tips will be better for the cobra rear bumper the 12inch or 18 inch with a 3.5 diameter and 2.5 inlet?