Hi Natchez,
I recently mounted that compressor on an F350,
I contacted ARB and received two replies, both saying that it is OK to mount it under the truck either to the frame or a cross member.
The compressor is double o-ring sealed and water and splash proof. Just don't want to run it under water or while actively driving in a dusty place such as when activating lockers unless you relocate the air intakes.
I mounted mine to a cross member using a mounting plate I fabricated from 1/8" steel.
Great compressor by the way. Check out the new compressor shoot out on Expidition Portal.
Mike