98exped4x4, pretty much it is behind the front tires or you are going to end up getting rid of the spare tire and mounting the tank there. Damned if you do, damned if you don't. The only other option is to get some 6" schedule 80 tubing and make your own tank. That will be way more wall thickness than you need so, even with rocks hitting the tank or someone T-boning the truck, the tank is going to remain intact. Then all you have to worry about is someone knocking off the port in the tank. Even then, you are only looking at a bit of noise, that's it. Then you can mount the tank to the frame rail and call it good. This is my plan if I get my new F-150. That way I can have about 10 gallons of air onboard and still not use up any of the bed space.
just for reference, using 6" pipe that is 5' long is going to result in a tank that is just over 5 gallons in size.