if you wind up redoing the transom one way or another, do yourself a big favor and create access to bilge areas and the check vales aft on both sides, replace it all while its open, but leave yourself room to work when you have to get back in. AND if you have the stupid livewell i have on my 87', make that part of the plan also. Those things addressed will save countless headaches!!That being said the shaft length is definately something to be addessed. If I was to do it I would completely close off and go w/ something like an armstrong bracket, or D&D bracket, many things to consider including height(length of shaft), attitude, as the boat may porpoise badly with the source of thrust being moved back, etc. If you are gonna open it up it might as well be done right. I've seen a few guys here, as well as classic mako and seacraft pull this one off. Do alot of reading, best of luck.
BTW, brackets nowadays come w/ a certain amount of flotation from what i understand, you don't want the transom sitting like a limp willie at rest for many reasons.