weld or rivet?

[asifnyc]

looking for opinions. I'm mounting my battery in the trunk using this battery tray. I have a mig welder and a hand riveter. If I use rivets I can only put them in across two-thirds of the bottom of the tray before I "hit" the gas tank underneath. If I weld I assume I need to remove the gas tank so I don't blow myself up right? Seems easier to just rivet it but will that be strong enough? maybe removing the gas tank won't be as big a hassle as I'm imagining? I don't have any help so removing the tank would be just me and a jack. I'm not sure how much gas is in it so I'm not sure how heavy it is right now...