Hello, I might be interested in your benelli 650 motor, I also know of a fellow with a Yamaha 650 motor so I'll follow that up. The shape of the benelli motor is a frustrating shape to fit in existing frames. The width of the wet sump at the front forces it too far back by the angle of the front down tubes and it is too compact for british featherbed style frames. I've looked for years to make a period 4 road racer ( the stock frame is inconsiderably heavy!) However during my search I identified that the Kawasaki z500/550 frame could probably be a neat fit. The front down tubes are wide enough to clear the sump and it is not too long.
Anyway I'll chase this bloke up with the Yamaha 650.
Best Regards, Brian.