[bigeasy1]

I'm in the process of installing a new transom this weekend,on my Tsunami;and will be doing the exact same thing you're talking about.
I bought the brass garboard transom drain assembly,that consists of the brass body,and brass plug.
The body part of the unit is a casting with a female pipe thread in the center,and a flange of approx.2-1/4 inches with three holes,for screws in it on the outside.
You just have to drill a hole of about 1" for the inside part to fit into,and the flange would sit on the outside of the glass.
The flange has three holes to put screws thru into the transom core.
I'm naturally going to have the edges of the hole sealed with resin,and will be bedding the casting into the wood with 3M 5200.I wouldn't want to depend on just the screws.

If you already have an existing transom with the drain with the brass sleeve,you'll either have to remove it completely,or you may be able to just cut the outside lip off and place the new drain inside,if it will fit.
The inside part of casting that is threaded,protrudes from the flange part about 3/4" and would be bedded into the hole in the transom.