I did mine on my old boat.
Not too tough- measure 5 times and cut once (4" hole saw). Before you cut, put masking tape over the gel, and drill a pilot hole in the center to be sure you are square. If not, you can always adjust 2" or so.
After you cut the holes, dry fit the tips. If everything is OK, put resin on the exposed wood.
Go buy some elbows, hose, and (double) stainless clamps, and you're good to go.
Getting the "Y" pipe off is another story- in most cases, you'll have to pull the engine to install a blockoff plate.