As I understand it the bore is the only difference; which is why the heads won't interchange. You'd end up with unevenly mated surfaces (sharp corners) which could wreak havoc with detonation, pre-detonation, shock-wave fronts, etc; but ask any machinist if it can be done easily. You should be able to overbore a 2.6 to a 3.0 or larger as long as you address the heads and check the piston to rod fit. Check out,
They over bore both a 2.6 and a 3.0 to a 3.6 with the same piston and rod set. I believe they mill the heads to accommodate the larger bore.