LOL, for a modification to be of any concern to an insurance company it must effect the original road handling and possible safety of the vehicle, example, lowering the suspension low profile wheels and so on, a tank bolted to the floor for safety purposes wont make the slightest difference to your insurance unless it has been bolted though something that might weaken the floor, even then the insurance company would have to prove that the damaged floor caused or added to the accident before it could refuse to pay out.
That said insurance companies aren't slow at putting useless nonsense on their policies to attract custom.