273 or 225?

I think the main issue that a person needs to know about a /6 before deciding to hop one up is the limitation of the head. Unported you'll be lucky to make 200 hp I bet a good chunk less.
Now if your willing to port 1 hp per cid is in reach, they can do 250-300+ NA HP but more than likely most won't and be more street strip/race engine. Now if that's ok build away.