http://www.rdsaluminum.com/homepage.html
I think it is page 18 of their catolog that has all there tanks. I called them and talked to them. They said their tanks are built tougher than any OE tank. Using them for gasoline is not legal in kalifornia (shocker). Also, you cannot let them gravity feed like you can diesel. You have to have a fuel pump which they said was readily available (12 volt in-line model). I checked at my local auto parts store and it is available and they can custom make the fuel lines as well. They have many configurations.
I was turned on to this company by my friend who has the tool box axillary tank combo. He has been happy with it.
I was considering doing the install when I learned that Nissan is SUPPOSED to be coming out with a diesel truck in the 09 model year, so I am holding my breath for that. I want to better mileage of a diesel and diesel fuel is subject to less degradation than gasoline is, not to mention the trucks are subject to less regulation for some reason. It could certainly be done though.
OH yea, they way they hook into the fuel system is at the filler neck of the OE tank. You put a check valve in there just above the incoming fuel line so that it can't spill. With a diesel you can just let the auxillary tank gravity the whole time you are on the road because it won't spill. This caused the check engine light in my friends Duramax to come on though. I guess with a gas truck you turn the fuel pump on, top it off, and keep going.
Let's see, what else? In gas trucks you have to have the vent on the top. You can vent down through the bottom of the bed with diesel. And there is a rollover valve on the breather in case the truck rolls, fuel will not leak out.
Hope this helps. It's all I can think of right now.