We got some real wise-guys here, don't we?

I don't have the measurements handy, but grab a tape measure and lay underneath the rear and you can measure it yourself quick as you can read it. The tank is rectangular and you can see all dimensions under the truck. You can put a larger-capacity cell easily within the frame rails. Just keep in mind that the stock tank has a top-mounted fuel pump that sits down in the tank, so you'll need to find a suitable frame-mounted unit that meets the required flow rate and pressure.