How big?
I plan on adding 1 small & 1 large hadly tanks for air horns & air. What I will be going to do soon is to pre mount them on a plate with the compressors, solenoids, etc. I would weld two bolt heads (to end up with 2 studs), on the frame inside drivers side near the transmission because it is wide open. This section of the frame is boxed or else I would have just used the bolts. By having a plug on the wires simple cleaning, repairs & changes can be made on the bench and not under the truck. hope this helped.