Im planning on redoing my entire exhaust system next week. I plan on putting on some headers that go into a y to come out as one single pipe, then putting an electric cutout before it goes back to the muffler.I found some headers online that have a 3 in collector but used a calculator online that said i should use a 2.5 inch pipe. how should i get from 3 inch collectors to a y coming out with one pipe. and what size pipes should i use from the collector to the y and then after the y.