My father had a 1964 Simca 1000 which was rear engine, rear wheel drive.
The gas tank was vertically mounted behind the rear seat and in front of the engine.
I think the rear axle was below, and a little to the rear of the gas tank.
Basically, passengers in the rear seat were leaning on the gas tank with a cushion and mounting brackets to separate them.
Better I suppose than a Jeep from Korean War vintage. The gas tank, with a cushion, was the driver's seat.