my condolences on your engine tino RIP 2.5ltr

it is my believe that's once you open up a subaru engine, the magic is gone. there is just something about it you can't open it up and put it back again exactly the way as it was.
therefore for my 2.5ltr upgrade, it would be a whole engine swap rather than a half cut. i assume that something in the 2.0ltr half would be restricting the 2.5ltr half once you put them together.

that's just opinion, but as pete said.........mod at your own risk!