I never measured it but there is some room between the front of hood and the radiator support, 5" i don't know.
I take it u don't have rear air?
If not how about a little bigger tank under the rear end where the factory compressor goes.
If u have a local truck parts store, there is a large selection of air tanks for HD trucks.
Another option is a truck junk yard and get one of the polished aluminum tanks like Peterbilt uses.
That would look sharp under the backend, balance out the polished skidplate up front
.
Also run the airline inside the frame like the rear mount winch does.