as above HT II , I think if you changed the internal spacer it would work am I right or are the cups somehow different ?

less seals (not that shimano really put many in)

The cup bodies are thinner, so you need to tweak the spacers. But once you've done that then they're fine. I got good life out of a dura-ace one on an mtb.

68mm shell is usually fine. 73mm shell and the threads are hardly touching.

I would have thought it would be the other way around. I assume that the total width has to remain constant so that the chainset is the same width apart and the splines sit fully together.

So with 68mm you would have more spacers than on a 73mm

it depends what fitment you get, as i tried a road BB and it was a completely different diameter/thread to the MTB BB shell.

Yep sorry I had it the wrong way round. If you need to use spacers (ie. 68mm) then there are very little threads left.