Pretty sure it was a 3/4 bronze mushroom. I dry fit it, measured and marked the threads and then cut it down with a hacksaw. Two man job to install. One guy holding the nut on the inside and one guy turning it with a step wrench on the outside. Pretty sure I sealed it with 5200. If I were you I would drill the hole and then fill the void around the hole with peanut butter. Let that set up and then use above method. This way when you go to install thru hull you won't compress the space.
http://www.perko.com/catalog/underwa...use_with_pipe/