IMO the BEST way to repair it would be to cut open a section of floor above , use various bits of timber and mallet to get back into shape the weld section of floor back in .
I would agree with this, a wide bolster chisel and a lump hammer in the corners would probably work well. I've seen them used to repair roofs where the car has been lifted up in a scrap yard and it was amazingly effective.
Sounds drastic, but the paint and any other protection is cracked now. If you pull it out it'll rust out soon enough on a well used car. At least, that's my experience and I have the welding tan to prove it. 😔
The problem you have is the that if you cut the floor out from above then attempt to dress the damage out you will still struggle to access all of the damaged areas - secondly the chances are that you will pull the floor down / close the box section up whilst doing it this way - the correct way in my book is to drill the spot welds out - cut the section either side of the damage and remove it - you can then access both sides of the box section to straighten it up with a hammer & dolly creating the original profile - once straight you can reweld it back in - plug weld your drilled spot weld holes & dress it back
1973 Rover 3500 P6 Auto - 23k miles - All original 1937 Jowett 8 HP Saloon - In many, many pieces 1951 Jowett Jupiter - In more pieces than the 8 HP 1952 Jowett Javelin - In regular use jowett.org/ 1954 Jowett Jupiter - Complete - future restoration project 2000 P38 Range Rover 4.0 SE - Daily / Tow vehicle
Flat Cap Classic Cars - Specialist Panel & Paint Restoration