Seems to me that moving the bed would be the easiest method. Here's the steps for the two methods I can remember from my 07 Silverado...
First - remove the tailgate, remove the rear lights, undo the light connectors and unthread them so they are hanging out from under the truck, remove the bolts around the filler cap and separate the inner area from the bed, remove the ground strap near the tank, then take out the 8 bolts on each side of the bed, slide the bed back a few feet.
Second - remove the bolts around the filler cap and separate the inner area from the bed, remove the ground strap near the tank, take out the front 7 of the bolts on each side of the bed, loosen the back 1 bolt on each side until only 2-3 threads are in on each, take a jack and a piece of wood to span the gap to the bottom of the bed, lift the front of the bed hinging on the back 2 bolts until enough room is available to reach over the frame and under the bed and get to the top of the fuel tank, then block the bed in the up position (don't trust the jack and wood with your arms in the scissor gap).