Just about any exhaust shop worthy of the name will either have the right flange in stock or at least have access to one. I doubt that the flange is actually welded on- - - -the pipe is probably swaged into the proper shape to seal the bottom of the donut and and then expanded to be a tight fit to the flange. The guy who builds my exhaust systems can do some amazing things with that hydraulic pipe bender. Of course, he's an old geezer like me with 50-something years of experience bending pipe.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!