Even doing that I'd have a plate on both sides of the transom just to be sure.
Fixed jack plate or any other jack plate would be a lot easier. You could dam off the remaining 5" with a number of things just using the existing top bolt holes.
Not necessarily this manufacturer but you get the idea.
http://www.marineengine.com/products...php?in=3428884
I'm not sure if a simple setback bracket will have the range of lift needed.
http://www.bobsmachine.com/Products/...k_brackets.cfm