Easy - the barrels are spaced far enough apart for simple proper straight stacks with correct bells [eliptical transition, full radius entry]. As tall as you like.dual IDF's
and they won't. But, the length matters. To get the sort of visual effect you are after, I suspect they will be long which is OK as that will lift mid range torque which is no bad thing on a 1300. If they have the proper bell shape, they will still be OK at the top end. But strictly speaking we are off topic as you are talking stacks on IDF not DCNF. I suggest you start a new thread around your engine cover/plenum/stacks/Ferrari/50s esq idea - which would look and be preety darn cool BTW!As long as they don't diminish performance,