I am looking at putting Stellings boxes on my Cafe when I repower...so I've been doing soem research on boxes for that application.
I would guess that the pivot point moves aft and probably the CG. I also think that unless you plan on closing up the transom, a nuetral box will not work. Moving the props aft has the same affect as lowering the X, and you'll be able to lose the spacers, but I dont think that will be enough...
Hopefully someone on the board has done this on a Scarab like yours and con offer more than my "I think..."
Good luck...let us know your results.