All spines have a 40cm long swingarm, from daytona to the last v11. All v11 swingarms have same measurments. NO extra stifner.
My daytona has a much more extreme geometry, feels quick steering but never out of control. It might be a combination of things, that make the first v11 stock not as good handeling as it could have been. What made a bad press to start with, and the new owners aprilia "solved" it. So made the frame 5 inch longer, a lazy headangle to name two.
They did some good things, the front engine triangle is stiffer, and the mounting between engine and gearbox/swingarm is a good one too.
But if you bother, why don't make a set lower frame rails, like a tonti. It's pretty easy, not sexy but I think it works, for all spines. You can find an example on Stan Friduss sport 1100.
http://www.guzzitech.com/racers/stan5.JPG