A stealthback in my opinion has always been aftermarket exhaust components up to the axleback exhaust.
Im pretty sure thats the way its always been. Whats being discussed here is just a catless downpipe basically. For that you will need a reducer as someone already said. Thats actually what Im running right now for my exhaust work.
If you get the mid and Y pipe in addition to the downpipe and cat-pipe, it will mount to your oem axle back.