do i really need 3" exhaust

I've got the single strap gas tank, big tires and a narrowed rear end with rubber brake hoses in the way. We are going to try to snake 3" tail pipes through there. I bought some flexible ducting at Home Depot and some expanding foam insulation, to build mock ups to send to the pipe bender. I'll start a thread when we get to that process. We are building an engine similar to the original poster's so my answer would be YES.