I bought flanges with stubs already attached from an online supplier and a handful of U-bends from Summit racing. If I were to do it again I would buy just blank flanges and weld the tubing inside. I had to cut a most of the stubs away due to the quick turn required.
I was looking at trying to find one with the stubs, but no one makes them for D ports that I found. So I already changed my mind on doing it with a blank flange.