An IC from a HSG would be the measurements you need, I'm sure one of the guys will have the size on hand for you.
Regards exhausts, there is no "one size fits all" arrangement out there, you'll have to cut and shut a few bits from other systems or use new bends. IMO keep the stock one, save your $ and get a good 3" done at a later stage.
Can you weld? Because that will save you ton of $ in the long run.
