They are basically the same frames, so they can be made to fit. It has been done often enough to the Nevada, (& the Bellagio, as well as the V9), although I don't remember seeing any Breva get the same treatment.
For the seat, you will need to fashion up a bracket for the rear end of the frame, to take the seat lock (these are available as a part) for the seat's oddy pin. You won't need to weld anything - there's two bolt holes on each side of the frame.
I'm not sure at the moment about the lip for the front of the seat, and the rear fixing point for the tank, and I don't remember exactly about the front donuts for the tank - I have the Breva frame (a statutory write off) in the shed and I can check for you once I get back from this little tiki tour - I have a Nevada frame, and a Lario frame I was going to use to house the Breva.