Perro,
I've done it both ways. I have the original behind-the-seat tank re-mounted at the front of the bed. It wasn't all that difficult . . . unless you're somewhat lazy. I sure like the cab room I now have. I have a second 25 gal, fuel tank mounted behind the rear axle with the filler opening at the back of the bed. You may have to use air-lift shocks to keep that fuel tank from dragging when full. They're cheap.
Good luck!
