Hi. I hope I will not be clouding the issue but many years ago I had a similar issue with my '68 trophy when I first put it together.
In a similar situation, i.e. changing into third and then backing off to change into top, the gearbox would "auto shift" into top.
After talking to my brother about he suggested that he would re-set the gear quadrant engagement, he did it "his" way, set the box in first and set it up like that, the same way a lot of other people have suggested here on the forum. I had tried to set it by the manual. The box shifted fine after my brother re-set it, and has done so for over 10.000 miles. By the way, looking at the picture of your gear cluster, it all looks to be in good condition.