The original wheel has a rim width of 6 inches, and uses a 5-on-4.5 bolt circle. You can find details in one of the "Switching to 15-inch tires" threads. If you are going to have a 5.5 inch rim width, be sure it will accept whatever tire you plan to run. The tire manufacturer web site will tell you. If you will be running a Goodyear Marathon in the size you mention, the Goodyear site seems to say you are OK.
http://www.goodyeartrucktires.com/pd...ce%20Guide.pdf
Bill