That setup ^^ looks good, but the valve mountinfg is suspect. Looks to only be held up on the 90deg fitting out of the tank. Will constanly be wanting to un-thread and put unwanted load on that fitting. I would mount them to the frame. otherwise good to go. And yes, you can fit two 480s where his dryer is, just mount them low enough to clear the bed parts there.
unless u r planning to hit alot of switches/ hop it, air horns, etc, u really dont need more than 10gals air. While it is better to have, its not necessary. I run dual 480s to one 5 gal but its just not quite enough to pick front and back up without waiting a few sec. 8 gal would be bare minimum.
The solonoid is the best choice like mentioned above, nice thick power wire, and just plug in the comps where u can into the one tank. (the leader hoses are not that long) But u can easily connect the two tanks together. on other trucks i did, i had plenty extra ports to run 1/2" airline on boths ends of the tank. (a jumper hose on each end of the tank)
Also, try Mac's Springs out in Highland CA. www.macssprings.com
or (800) 585.5117 Talk to Craig, John or Hasson they will do you right