I would think swapping the complete 650 would be the easiest. But if you do decide to use the 550 bottom end and the 650 top end, use the 650 cams. When using the 650 cams you will need to switch the timing gears off the 550 cams to get it to work.
I believe mac 4 Into 1 exhaust is all they sell anymore for the 550/650.
Also the CV carbs are more difficult to tune, but are tunable. I tuned my 550 with cv's and velocity stacks, and it ran rather well after playing with some different jet sizes.