yes, it definitely does matter. anything bigger than the actual outlet diameter of the manifold flange doesn't help flow a bit. the 1-3/4 to 1-7/8" hole in the manifold flange is a bottleneck!!!! It doesn't take much with a die grinder and burr, to open the manifold opening. Don't worry, there's lots of meat there// you won't be "too thin" but it DOES make a difference!!!!
Rusty Rat Rod had a thread with pix about this very thing, about a year ago right here........
and the last one I did that treatment to, as cast, the exhaust opening was way off center, compared to a flange gasket bolted up loosely as a guide// so that gives you a chance to "square things up" at the same time. (most of the flange gaskets I have found with that bolt spacing are a 2-1/4" hole, so that's as far as I grind)
