The outer skin is already there, its your hull, measure that, then add your total wood thickness, then add another 1/4" for the new inner skin you will be covering the inner transom with and the pb left as glue..that equals the total thickness, and shouldn't be more than 2 1/4" and not more than 1/8 deviation around the keyhole. If you need to do some fairing, incase the parallelism is off more than 1/8, that adds more...so if you remove the entire 1/2" smaller piece, that leaves roughly 1 1/2" of wood, , then 1/4-3/8 for the hull thickness around the keyhole, another 1/4 for the new inner glass, and 1/8th for the pb glue between the wood and outer skin...and a little room for fairing.
Your doing fine, im just hammering this point cuz its mucho importante....it would be a shame to get all the way to the drive install, and have steering arm clearance issues...or weak transom shield bolts.