Hey Ed,
When you cut the rest of the factory bracket off, the bolt in your pic that is welded is threaded into a factory threaded insert in the frame. I would measure from that threaded insert since it has always been part of the frame.
Then you can verify it by measuring off a couple other reference points to make you feel better.
You have some wiggle room in the hiem threads for adjustment incase your frame is not square or other slight imperfections.
Mike