Nothing I know of is available. From what I have seen with the "exhaust through the hub" lower units, it is unnecessary. The early V4 75 horse used a huge tapered prop nut for a smooth flow of water. No change in performance was found with the use of a smaller nut. The biggest improvement was in prop design. The only way to ensure a smooth transition would be to use a longer prop shaft. Then a transition piece would be more practical.
You may be assured that if I am wrong I will quickly be corrected . . . 😆
