I can't see a reason why it shouldn't be, the only difference I can seem to remember is the 2.6 has 1 carb and the 3.0 has 2 carbs, but it is years n years since I've seen a 6 cylinder Landy, so I could be talking total bo*****ks.
The Weslake head on either the 2.6 (Rover P4 110 engine) or 3 litre won't fit without modification to the clutch pedal box. However, the early non-Weslake 3 litre drops straight in." HTSH