I'm thinking about using a qwikrete buildup to replace the old sill. The problem is that it would have to span the garage slab, and the slab on the outside of the door, and any differential movement between the two is likely to cause a break. We have swelling clays here and movement is probably what caused the old sill let water through.
Any ideas on the best way to proceed? A couple pics in the link below show the situation.