Andrew - I had a pair of Carlisle 26-12.00X12's on my 982, but they were a different 26-12.00 than most people run, they seemed the same size or just a bit larger than my Firestones I have now.
Maybe somebody more familiar with the Carlisles can comment on how they compare size wise to the F'stones.
IH spaced the fender pans up only about an inch to fit the 26-12.00's, if you mount the tires and decide you need more clearance, you could just put a couple flat washers under the attaching bolts, think there's six of them... maybe need to use 1/4" longer bolts too.
Guy I bought my 982 from used a pair of 26-12.00's F'stones on his 782 with no modifications, the lugs got close to the fender lips, but no rubbing.