Good idea on on removing the adjustable David. That will certainly do the trick.
And Chemist, with the door being that old, it certainly confirms again that; "Thay don't they use to anymore". Regardless, I suspect whatever is hold the sill to jambs will be cut very easily with you Multi Max.
I certainly agree with David's approach. The only set back I see with that is getting a prpoer seal with the sill to the concrete. Because it has to slide in so snug, I just question how well the caulking will spread.
On the otherhand the option of taking eveything out and installing the sill to the jamb then tipping the door back in place will certainly seal well.
But considering the age of the door, any caulking seal will at last it.
I do have one question: Do you have constant water splashing up against the door?? Up my way, I can just about guarantee that every house that has a granite slab, or a precast stairway has either caused severe water damage to the door in a very short period or it'ss in the process of happening. These type of scenerio's are constant cause of door sill rot or damage. Here's a perfect example: Check out the brick steps