I'm afraid whoever welded that up should cut their tools in half so they fit better in the bin, you are going to need to do a fair bit to put it right but at least once done you will know its solid.
I'm sure others will be along to say the same, the sills really need to be changed for the correct short type to avoid future rusting problems and also retain the strength of the shell. Given the standard of the other work you don't know what is under the sills and they might have left the old rusty ones on so getting them off will ensure thaty you don't have any future issues.
Fortunatly the poor welding will make removing them easier, a flap wheel in an angle grinder and some thin cut discs will make short work of it.
Best of luck, its going to be a great project.