Maybe a rectangular plate with a suitably drilled and tapped hole. You could hold the place on a magnet while someone else threaded the bolt in.
This way you wouldn't have to try and clean the inner sill up for welding and if it is rusty it will spread the load. If you do need to remove the bolt and you have worked out the correct size of the plate, it will become trapped and won't turn when you remove the bolt.
I suspect that these bolts are fitted with loctite, to remove them successfully the bolt needs to be heated first to soften the loctite. I work quite a bit on Harleys and Triumphs, almost all the bolts are fitted with locktite, any allen or torx bolts that are not heated first will strip, I always heat them first before attempting to undo them, saves a lot of time and hassle in the long run.