Right, I had a play about yesterday with the fatty’s brakes, didn’t get the trail out so this may be wrong, but:
The Fatty comes as standard with Is mounts and a IS-Post +20mm adapter, Avid DB3’s, a 160mm disk and NO conical washers. My other Avid brakes (elixirs) look to have identical calipers but with **5 rotors.
I swapped to shimano brakes and 180mm rotors and have the same problem as the OP. Tried it with all the ~185mm addapters I had and none would work, all put the caliper ~2mm too high. An avid 185mm rotor fit perfectly on all bar the Hope ‘B’ adapter which is a nominal 183mm. Even a +0 IS to post adapter mated to a 203mm post-post front adapter was too tall.
So, a bit of googling suggests shimano adapters are 180mm, so that’s the next thing to try.
In terms of helping the OP, I think the only solution is to try a shimano post-post 180mm adapter and matching disk, and if that doesn’t work aftermarket 165mm or 185mm rotors.