Now working on a PC I see the images a bit bigger. I now also notice the vertical side is buckled, I missed that at first working of my phone.
That will be a bit more tricky to correct. Getting something inside (an anvil/dolly of some sort) is the key to shape the steel.
wrt Bavbob's suggestion; If you choose this route, then i would drill 2 or 3 small holes through the bashed seam, before separating the lot. After all is straight, you can then use the drilled holes to drive a screw through (or use a CLECO). This will ensure the horizontal and vertical surface will go back to their original positions relative to each other.
Bending the horizontal surface up & back and getting it flush again may not be easy: This is the part that is actually not (really) deformed now, and thus it helps you now to keep everything in place (keeps the top of the vertical plane in place when you bash it); it is your 'reference' so to say. Deform it and your reference may be off.
Some other suggestions:
- You could also cut open a small 'letter box' shaped opening on the horizontal surface, to allow you to access the back of the vertical panel. If you position this 'letter-box' opening carefully, it will be fully hidden by the seat rails. It allows also to hold something behind it when you hit it. You may need to fabricate something with a steel tube and a block.
or
- (and this is more cumbersome, but it will leave most of your vehicle intact): Fabricate a tool that will allow you to hold a solid steel block behind the buckled wall, and slide it in through the seat release opening (need to dismount it first). Think about an L-shape: two tubes at 90 degree + a steel block welded to the end. With about 500 gram (a pound in US terms) of mass behind the area where you'll be hitting it. This will help you tremendously in getting it all back where it belongs. Given the room to play with, I think you may not reach all the way in the back with such a tool.
Regarding cutting & welding; mind you that if you should want to weld the horizontal piece to the sill, it will give burn marks inside your sill, that you cannot treat effectively. They will rust from day one....)
Between these suggestions I'm sure there is something that will get you started.
Still, organized basking will be quicker & easier compared to cut & weld.