Boyd Welding makes some nice aluminum tanks. Not that the Porterbuilt isn't, as I offer that as well, but the Boyd is a little more affordable depending on your budget. The Boyd tank hangs down below the frame rail about 2.5", but this can be modified if you want it tucked up higher. Just need to adjust the size of the tank and/or clearance the bed brace. You end up seeing about 1.5" of the tank when you are standing behind a lowered truck.
As they come, it doesn't require any cutting of the bed or anything, and is a bolt-in deal. Just depends if you go with the bed fill version or side fill.
Hope this info helps.
Boyd Tanks - $349 (carb version)