Thanks guys. Actually knowing that someone has installed a configuration and has gotten it to work helps a lot.
No worries about the 18s vs. 20s WLDHRSE. I've looked at the Harley Editions from time to time so I knew the F250 could at least arrive from the factory with 20s (though I didn't know they were 8" wide).
Here's what I'm thinking now based on everyone's advice. First, I measured my garage and I have a little over 9" of clearance left (somewhat more than I'd thought just by eyeballing it). Instinct tells me that going from 18s to 20s, from 275/65 to 285/65 and adding a 2"-2.5" leveling kit shouldn't increase my truck's height more than that. Anyone agree?
Not knowing exactly how much offset was required to get the look I wanted led me to more offset than my cattle ranch truck would probably look good with. I'm thinking a -12 offset will look great which means I'm back to some Moto Metal MO950's instead of the KMC Diesels (did I mention this process seems to never end). No big change there except that the MO950s appear to be in stock whereas the Diesels are backordered.
BFG AT KOs don't appear to come in 35x12.5x20. The closest I can find are 35x11.5x20 (LT285/65R20). I'm not sure why TireRack.com didn't suggest this in the first place, but 4wheelparts.com has them and the MO950s so I'll probably go with them.
Last (thanks for your patience with the rookie) comes the leveling kit that I'm now going to add to this setup. I've actually looked at these in the past because I think the brush guard I installed sags the front end too much to look good. Trouble is, these kits seem to vary wildly in price. Is there really anything different between three options like the ones below?
Thanks again guys. BTW, mohlson, your new ride is sick. Nice upgrade!