Starter motor replacement

So today I made attempt to remove the starter motor. A nasty job as I don't have car lift (yet) and I need to work on the dirty ground (and I don't have the possibility to move the car either. So I used the jack and a small ramp to access it:

The new starter motor with only 2 holes to mount it:

The bolts of the starter motor are difficult the reach, well mainly the top two ones. Yes, there was a third one and very difficult to reach.

I was able to get the first two bolts removed. The bottom one was easy. The top one was doable and came off fairly easy - though I could make only very short turns, so it took ages to get it off. The third one was severely stuck. To get it moving I had to use this setup using another jack to push the wrench up.  Luckily it came off - again it took ages as I could only make very small turns.

However the starter wouldn't come off. It looks it's stuck due to a "bus" on the thread end. So I need to get the thread removed. So for this I need to source another 5/8 inch bolt … sigh

With the "bus" I mean this "lookalike" spacer. But trust me, it does not move. I have been hitting it with a screwdriver and a hammer. The top was mounted with two bolts and the bottom end with this thread and nut.

I have been hitting the starter motor with a soft hammer, used a long screwdriver using it as a crowbar. But it just wont move. Almost looks like it's being held together by this thread. So hence I'm going to attempt to remove the thread once I have obtained another 5/8 inch nut.

I also tried to remove the cap on the starter motor but I had two "bits" which simply broke since it was that stuck, sigh!