I've got 235/75R15's on my stock 15" rims. (I've got a 3/4 ton)
That comes out to roughly 9.25" wide. They work fine on the front fenders, but I've got a flatbed right now, so I can't speak for the rear ones. They're only a couple of inches taller than the old 7.00x15's that came with the truck originally, and I'm thinking that they'll be fine.
The thing that you have to remember is that the tires (and the wheels) will be partly over the drum, so it's not that you have to worry about width so much (just so that they're not rubbing on anything or hindering steering) it's height.
If you measure from the center of the hub to the top of the fender (and then subract a few inches) you'll have the radius of the tallest tire that you can cram under there.
