I agree with Schlotzky although I'd call it a spark shield instead of a heat shield. Heat won't be an issue that far away from the tank. You just don't want a spark to find that leaky o-ring on top of the tank letting the fumes out. If you don't have a piece of sheet metal to use as a shield, you can use a wet towel or canvas tarp and make a curtain between the weld and the tank. I worked in a welding shop where we made custom trailer hitches for motor homes and I've welded within an inch of gas tanks (barely enough room to get the stinger in there). The first time makes ya really nervous. After that, you get used to it. Just take proper precautions. If you smell any gas fumes, don't spark up.
Oh, and, have a fire extinguisher or at the very least a garden hose near-by.
2002 WJ Overland - 4.7HO - 231tc - 6" Clayton - 285/75-16 Cooper STT - ProComp 5129 wheels - Kevin's Offroad Belly Skid and Rock Sliders - JKS Track Bar and Disconnects - ProtoFab bumpers