1200 miles is good, that's more than I normally do although I'll admit I only ever use my old cars when the sun's shining... which is sometimes "challenging" in the UK
I wonder if your gearbox problems are shift-related ? With my shift mechanism there's not very much you can do apart from the middle joint dimensions but I wonder if there's any tweaking you can do with the arm lengths on yours ? It just sounds odd if there's no untoward grumbling from the box and it shifts down ok, it makes me wonder if the linkage isn't quite spot on.
But even so, I'd say you're doing quite well with such a small "snag sheet".
Brian