On one of my trucks I mounted the air compressor where the ECM for the auto transmission would mount..... passenger side fire wall... there are 3 dimples that I punched out and made a mounting plate...
On my 04, and newer trucks I used the pacbrake mounting plate above the injection pump, and put the air tank under the floor on the drivers side of the frame... ...
We also mounted the air horns behind the front bumper... .
Watch the mounts for the air bags..... on my 3 trucks we found that the brackets for the air bags were not built well enough and we had I think the drivers side give us problems..... we had to reinforce them after 15000 miles or so... ... . we use the air bags to level the load and often have them at full pressure... ...
We later built a 2x2 square tube, 3" long and extended the bumper for the overloads so they would touch and come into play as the truck was loaded... With the air bags the overloads never touched on our 4x4 dually's... ...
Hope this helps...