You want to go to www.flitelinesolutions.com The information there in good and very helpful. Some of the Fox reps' can't really help unless they are there with you and your engine.
But it still sounds like you have the idle to rich. At least that's what it sounds like to me. What is the enigine mounted in? Upright, side or inverted installation?
An 11x8 will work, but will work better with a tuned pipe, the .74s' that I've used where happier with a 12x6 or 13x6 with the stock exhaust.