I already have a new flange, I went ahead and got 2-1/4. There's several exhaust shops near me and every time I want custom exhaust made these idiots want to leave as much of the original as they can, like cut the original Y pipe and leave a foot or more and when I say no, you're gonna replace it all the way to the manifold, they tell me they don't have any flanges, I've had to bring my own before.
And riddle me this/ with the manifold outlet at 2-1/4 (which at this point would be a choke point even though I opened it up "that far" as it is... I don't see how running any bigger pipe than the manifold opening would flow any better. On a v type engine where there is a Y pipe, I can see each branch terminating into a larger single line would be a help, but with a single outlet how would it help to have it any bigger than it is right there at the outlet. (Maybe a dumb question/observation?) But I don't see it being any better going with 2-1/2 pipe butted up to a 2-1/4" hole?