I put the viair system on my truck with a constant 150 psi. It was a bitch to run all the plumbing and find room for the air tank. They put all the bungs in the wrong place seeming how you want to hide the tank under the bed. Air horn is next. Ebay seems to have some of the best deals out there. But don't waist your money unless you do a good pump and tank system.
And dont believe the bull that the guys on Ebay say about how easy it is to mount, make sure you have alot of room!!! These horns are big! IE short bet f250 is a real nightmare to mount.