Got a couple hours to play around with this today...
It looks like I can make the 550 wheel fit on the 450 swinger. Need to go to a 110 tire from a 120, it was REALLY tight with the wheel full forward. Like 1mm clearance. Also, need to take a few mm off the face of the sprocket holder. the 450 sprocket carrier is shorter than the 550, but not the correct diameter to fit on the 550 hub. Looks like with the sprocket set to match the depth of the 450 carrier chain alignment will be spot on, with plenty of room for the disc brake on the other side.
Did the front end swap, easiest it gets I think haha. Both had roller bearings which looked like new, in the same size, with the same length stem. So, I made sure they were greased up and slid the 550 front end on. With the bump to a 19" wheel and the slightly longer forks, the front end got raised 1.5" or so. I'm going to push the forks up a bit in the trees for now, but in the future they will be properly lowered with the cut spring and internal spacer method.