Eyeballing it you may think that it would fit but having taken both of these bikes down to the frame and back up, there is a significant difference between the two.
I am fairly certain that the 750 engine is too big for the 550 cradle without some major modification. The dimensions you need to check would be (at a minimum) the width of the bottom of the cases against the bottom tubes, front case mounts against the downtube mounts, bottom of cases to top of valve cover to bottom tubes and top tube clearnace (almost no way you'd be able to get it in without cutting something. There's also the question of lining up the final drive to match the rear wheel.
Not saying it can't be done but it's not going to be a plug and play job. You're not going to get the same handling due to the additional weight and changed weight distribution. That being said, I'd be interested in hearing about it if you decide to go for it.
