I don't have experience building one but we do have one on our Hitchhiker. There's a certain angle the shaft needs to come off the gearbox
at (<7 degrees) and of course a certain depth
for the prop so there's no cavitation. This can mean a l-o-n-g shaft...I could send pics of our set up if you're interested.
I've seen Easton cats with a long shaft from an engine
in one hull
. Could ask them how it's done. Contact details: Easton Cats: Homepage