What a bugger. They must have put that starter on with an impact wrench.
That top bolt.

I tried every method for getting that bolt loose that I could find on the net and nothing worked.
Here's what I did.

I got a 10mm combination open end and 12 point wrench.
Got it secured to the top bolt.
Got a piece of 1x2 about 18" long.
Got a bottle jack.
Put the 1x2 under the open end of the wrench.
Put the bottle jack under the 1x2.
Jacked the bottle jack until I head a distinct crack.
I was then able to undo that cussed bolt the rest of the way, tiny bit by bit using the 12point end of the wrench.
Removed the starter.

Soaked the bolts in some oil.
Put the new starter in and tightened the bolts. Had to back out the bottom two a bit and wiggle the starter and then tighten the upper bolt first.
Invented some new 4 letter words along the way.

It might not work for everyone, but it did work for me.

Good luck
Greg