I built a custom tank for my Isuzu. It held over 40 gallons of gasoline. It was mounted in the back and between the frame rails. I never had an issue with this arrangement. Couple this with a solid steel bumper, an excessive skid plate for the already 1/4" thick gas tank, I don't think any vehicle on the road today could puncture my gas tank, unless they were traveling at missile velocity or were significantly larger than me such as a 25 ton semi. Then at that point I think survivability would be a non issue as the difference between being compacted into a 2'x3' square verses being compacted into a 2'x3' square then catching on fire is mute point.
I say let everyone else crumple.
Besides, I'd much rather have the weight of the gasoline centered over the middle of the vehicle than only on one side such as is the case now with the factory tank.
Plus added weight just behind the rear axle will add down force on the rear axle and provide more traction on the rear tires.
And yes, excessive weight the further away from the rear tires could make it easier to flip over backwards, unless you provided some sort of rear stinger to stop you from flipping so easily.
I'm going to be building my rear tank as a custom project. I'll be able to hold more gas than all of you and go on longer desert excursions without the need to have more gas cans.
So in other words, you haven't convinced me this is a bad idea yet.