Id join it a touch higher then the swage line otherwise youll have to grind down all the welds to much so you can keep the swage the way it is at the mo.
Id set it up as if you were doing a but joint but put a strip of steel at the back of the joint to give it strength and something to weld to.
hold the strip or steel to the original sill by welding it or screw
it in place then remove them and weld up the holes afterwards
