I don't 100% agree with removing the flappers. If you do, I'd look into removing the exhaust bellows and using an inboard style flapper on the exhaust opening on the gimble housing, just like inboard boats have.
You are over at ifourwinns, right? There was a guy over there (deafwish) who had done this on a Volvo, these flappers come in differnt diameters, you'd have to measure the diameter of the exhaust opening in the gimble housing to get the right one.
like this:
