Mr roohairy, welcome to boatmad.
My personal opinion of the GM/AM General V8 diesel is that it's a nice motor, bit heavy, but a nice motor, and if you're only looking for 225hp it will be a very clean running, torquey piece of kit.
The Stern Pwer drive, whilst a little agricultural, will I'm sure be fine, but at 225hp so would a Bravo1.
My only reservation for you would be, unless you're very competent in the engine building/marine engineering department, converting an automotive 6.5 GM engine (higher compression ratio) which has an unknown history, is a risky business.
My first experience with the 6.5 GM was an engine from Justin @ Samurai, all I will say is, it took me a long time to get things right with that motor (it was about 230hp, turbo, no intercooling) and I think I understand engines reasonably well.
In short, I would get a 'Marine version' of the GM unless you're in the know.
PM me if you want to talk in more detail.
"I Agree with everything you say really!" - John Cooke to Jon Fuller - 26-01-2013