when i first started my "train horn" project, i started with a 5 gal. tank mounted on the frame just under the rear pass. door, had to drill new holes to match up. i had a set of triple chrome "china" made horns that i mounted up inside the front part of the bed area on the underside of the pass. side.
i had to do some riggin to make it work, but it was ok. i didn't have compressor at the time, so i had to refill the tank at a gas station, but could only get about 80 lbs. in the tank.
i finally stepped up later and bought a ViAir continoues duty compressor and an additional 15 gal. tank. i hooked both tanks up together and mounted the 15 gal. tank and compressor in my
TOOLBOX, and left the 5 gal. on the frame.
this combonation has proved to be just right for me. i also stubbed out a couple of quick connections to run an air hose off of, incase i needed to air up any tires on my truck or trailer.
for my on/off switch, i used a
HEAVYDUTY AIR SWITCH, to accuate the loudness. so no solenoid/electrical switch to open up the airways. i gotta 150psi right at my fingertips.
let me know if i can help in anyway.