Nail,
I am afraid you are stuck with those unless you get rims. The 550 came with the following tire sizes:
11.2/10-28
12.4/11-28
13.6/12.26
13.6/12-28 These are what I have
14.9/13-24
14.9/13-26 These might work

My are Firestones and I think they have the 13.6/12-26.