The transom thickness needs to be 1.75"-2.25" (if my memory serves me right) as the two halves of the transom assy (inner & outer castings) have the drive shaft going through them at 12 degrees, a transom outide these dimensions will put the alignment out.
So your ali plate will work, as long as you pack the thickness up to 2" with ply, which would also give a little compression when the thing is tightened up, which would be a good thing.