Welcome, she certainly looks to be very neat elsewhere.
Quite an unusual area to have damage. Like others have said, pull the tank and look for damage or repairs under the rear end. The tank may show damage or have been replaced. Look around the rear fenders, down below where the exhaust splitter tips exit, around the trunk drop-off area and inside the trunk walls for any ripples or fender repairs. Hard to say if it has been driven off or over something that has forced the tank upwards or even if it has been brought down on a hoist with something under the car they forgot. Marks on the underside will tell more perhaps.
If confined to that area only then a bonus. You can get new and used trunk floor sections now too, should it be too hard and too much work to knock into shape, may be easier to replace that section, the rest of the floor may also be bowed upwards slightly. Even look around the roof and window areas for signs of ripples or repairs as these cars flex easily and that's been quite a bump, moreso the T-top cars that flex and buckle just from driving. Do you know how recent the paint is?
