Come on Howard- That's a good looking repair in anybody's book. Look where you started and look at the finished product. When I was in the USN I was aboard the Jesse Brown a Knox class FF that had an aluminum superstructure welded to a steel main. With salt and electrolasis you could wipe the white pouder off by the hand-full. Yauk. I don't know of a good way to flush your transom. I was thinking about flushing a radiator with a hole in the top and a hole in the bottom on opposite sides. Sounds like a lot of trouble. Maybe somebody else has an idea?
tccanoe- As you can see I was on a bird farm, USS JFK...... I also know very well what salt water can do, LOL, I found entry points on the inside top of the transom and it is not sealed along the entire bottom so it will be easy to flush.