Ford E-350 Van's come with 245/75R16's. Ford E-450's come with 225/75R16's. Will the 245 work ? Sure, but if Ford thought it better to Spec the 225 on the dually chassis shouldn't you follow suit ? You're call obviously.
My input though would also be to look at other tires. I owned a bus company for 15 years. We had buses of all sizes but our smallest were Ford E-350 15 passenger Vans. Those units all came stock from Ford with the Wrangler HT's and we absolutely hated the tire. Terrible in the rain and snow (especially after 20% wear or so). Mushy ride (soft sidewalls) and they also wore out very quickly. The Michelin's (LTX M/S's) we always replaced them with could easily give us 60,000+ miles where the Wranglers were toast in 25-30,000.
I'm not positive but I'm pretty sure that all of the E-450 Mini-buses (24 ish passenger) that we purchased over the years (same chassis as the Greyhawk) came from Ford with Michelin's.
Visited in our 2011 Greyhawk 31DS