Why not forget the transom housing and make a race plate?
http://www.hi-techperformance.com/images/9856.jpg
http://www.hi-techperformance.com/images/9857.jpg
http://www.hi-techperformance.com/images/9858.jpg
It cleans up the boat and makes the pump soooo easy to work on. Stops having to deal with all of that silicone every time you want to work on the pump, and you can change an impeller or wear ring in 20 minutes or less. All it takes is a 3/16" piece of aluminum.