Not 100% sure off the top of my head, but you could most likely run that wheel off the 350 and use the axle from your 550. I believe that the triples on the 350 are a bit narrower than the 550. You might have to have a spacer made but that's probably about it. You could use whatever spacer you have left over from the swap and cut/machine it to the correct width.
That 350 have a drum or disc hub ?