Every once in a while we have a case where the depth of the factory fuel tank is more shallow than expected on this platform.  (5th Gen Camaro, CTS-V2, Caprice PPV, SS sedan, G8, Holden Commodore and Maloo)

In this case, the fuel tank is too shallow, so it needs to be corrected.  The procedure sounds scary, but once you've done it once, it is easier the second time.  We ONLY recommend this procedure for experienced shops.  Of course, take all precautions to minimize ignition of the fuel, and have all safety equipment accessible, since you will likely be doing this away from the building in the parking lot.

Essentially, you need to heat the bottom of the tank and apply pressure to push it into spec.  We've done it with the tank flipped upside down, a short 4x4, a propane torch, and a bucket of water.

With the tank upside down and the 4x4 inside the tank opening (one end on the ground, the other end on the bottom of the tank) heat the bottom of the tank (from the outside, of course) for a minute or two.  Then apply force downwards against the 4x4 so it expands the bottom of the tank.  While holding pressure, have somebody else poor water on the outside of the tank to cool it.  Then re-measure.  Your first iteration will probably not be enough movement.  Your next iteration, you'll probably overshoot, but that is okay.

When you get back to the car, you'll have to extend the tank strap with some washers.  We think short tank straps is what causes the tanks to sometimes be short.  I'm a fan of cutting the floorpan and installing an access panel, so the fuel pump assembly can be serviced without dropping the tank down again.  Additionally, this ensures the pump assembly is installed without any force pressing on the bottom of the pumps.