How would you manage to weld all the existing parts to the new T piece? They all need to be connected for strength I assume.
Apparently looks like this
So there should be 3 layers in the sandwich, flange on floor, stiffener and outer sill, the inner sill is flanged to the floor.
If this is a proper keeper car (which it seems to be) you should do a proper job, remove the outer sill, and stiffener, the cut back into the inner sill and floor enough to weld on new flange pieces, them replace the stiffener (which looks just flat, maybe just weld a new bottom edge on), then replace the outer sill (again you might be able to just weld a new lower flange on it.