Split 999 your pic there is exactly what i need to do, did you then drill holes in the joggled edge of the new panel and wled from the inside or did you weld the top edge of the joggled panel, and will you finally weld all the way along from inside or outside.
Yep as Mark says....weld all the way along on the outside until you have a continual line of beads with no gaps. When the gaps get to about 3 inches wide, this is where you really slow down. Let the panel cool between EVERY
spot weld before doing another. "Cool to touch" is a good guide. If you stick to this you can achieve very little or no distortion. Also make sure the bottom of the new sill panel lines nicely up with the inner sill. When you're happy with alignment, be sure to apply grips to the bottom lip all the way along. If you don't clamp it back it may lift off when you're welding along the line. You don't want that!
On the inside you will see that the joggle presents the perfect water trap for condensation on the inside of the panel. You need to seal this with a good sealant, but prime first then apply the sealant. I used epoxy primer which happily accepts sealants and fillers on top of it. Sikaflex 221 is a good sealant to use.
Hope this helps.