According to Goodyear's site a 9" rim is within spec:
Personally on that wide of a tire a 9" rim seems narrow to me but Goodyear says its within the approved size. A wider rim will of course give you better wear and footprint on the tire because it won't be bowed in so much. IMO I'd go with the 10" rims (or even 11" per Goodyears site at that is the rim size they used for the measurements) if you need the extra footprint that would offer and if you're not talking a lot more $$ per rim...
Post up some pics!!