I hope you know that you can 'overlap' a smaller tube inside a large one (using about 2 x D for the overlap), position and attach the tubes with a pair of centering rings, and use a cardboard shroud to form the exposed surface of the shoulder. You can cover with a little glass if you feel the need.
Or do you have a special need to use the actual transition part from Performance? (I'm sure they have great stuff, not trying to knock any of their parts, just offering a thought in case you want to keep on building and not wait for a delivery)