I just found your thread and wondered how your boat project was going. I own an Excalibur 26 also. We sailed it for three years and then I built a trailer for it and drug it home to restore. I have done some accomodation modifications to make it more user friendly for a family of four, and am still working on it.
As far as the motor well goes, when we first bought the boat it had and old long shaft 10 hp in there. When that died I installed an outboard mount like you have on the transom to use a short shaft merc I already owned. The transom mount is next to impossible to use because I had to crawl over the back of the cockpit and lean down to get the controls, and to raise/lower the thing. You can't reach the tiller and motor at the same time, and leaning over the transom is not a safe place to be when motoring out of a tight harbor in rolling swells. Also, the transom mounted engine will come out of the water one minute and be submerged the next as your boat pitches up and down.
I would suggest mounting the longest shaft engine you could buy in the motor well.