Unfortunately, you can't easily build a Mauretania from a Lusitania kit. They're sisters mostly in name only, built to the same design brief but with many minor to moderate differences by different yards. Cunard was essentially putting the two yards against each other to try to get the best ships possible at the best cost. They were dimensionally different, too, even though on paper they shouldn't be. Is it enough to make a major difference, those few inches? That's up to the modeler.

I really want a 350th Mauretania, but I fear I never will see one, sigh...