I am about to weld in my new sills, as my other thread clarified the questions I had.
However, the flange that the sill spot welds to is rusty. Not all the way along, but there are a few parts that are completely shot. I have stripped back the paint on the doorstep, and the is no filler and no rust on the bottom of the actual doorstep or rear quarter-just the flange.
How would you repair this? Welding on a new flange wouldn't look too good, as the weld would build up. My other idea was to make a 90 degree piece to weld behind the doorstep/rear quarter. Here is a highly technical drawing;

This would mean there would be no weld build up on the outside, and plug welds would be ground down and skimmed after welding the sill on.
What do you think? Would it just trap water and rust?
Thanks