The lower mount has captive nuts inside the door pillar.

Mine had rusted up to the point if no return. In fact they snapped!

I couldn't see any other fix apart from cutting into the pillar.

BEWARE! The foot of the bulkhead has a U-section member that extends up the pillar well past the door mount. This means 2 layers of steel whilch may present problems when stitching up again. You can see two thin cuts I made near the hinge mounting holes but I stopped when I saw the heavy guage steel underneath

I thus elected to cut into the pillar from the INSIDE the post, about where the driver's right ankle would be. This side is non-structural & has only one thin layer of steel.
Then I made a piece of heavy angle with two nuts welded to it similar to the top hinge arrangement. BUT how to suspend it there as it can't be reached? Captive nuts were too hard so I used a short length of chain welded to the the nut bar (!?) & attached to a 3/16 setscrew. I drilled a hole for the setscrew so that the nut bar hung just below the holes but was easily lined up with a Phillips head screwdriver.

Then the setscrew was welded in & ground off and the hole patched.

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