Can I replace an Ultegra 6700 HTII BB with a Dura-Ace 7900 unit? Threads for shell are obviously the same. Just wondering if there is any difference in the length of the spindles on the 2 model cranks which may mean different width BB cups? A lot of places sell both for the same price so might as well have a Dura-Ace, though I suppose there is little difference if the price is the same? Can't afford an alternative such as Hope/CK being jobless. Gonna try to relube the current bearings first. Just wondering in case I knack them.

You can, yes.