Well, no expert opinions, so.... you get *my* opinion, FWIW ... packaging a gas tank and a battery (lead acid, I assume?) together in a metal box pretty much sounds like an IED to me ....
I'd go with the open bottom at least. Do the gas tank and the battery *have* to live together?? Seems to me that on most production vehicles I am familiar with, the battery and the gas tank live on opposite ends of the vehicle .... Probably a reason for that ... Just sayin'