Exhaust Nut

Can a 750 exhaust nut be used on an 850 exhaust with crossover?

I don't think so, the 850 nuts are shorter, and they have a larger OD that accepts the split collars .

Not sure about there being any difference in internal diameter (on mine, at least) - I shortened a pair of Mick Hemmings bronze collars by turning 1/8" off to get the right length for a Mk11A exhaust setup and they work fine. The later style 'one-piece' (split, but fixed together with a joiner) retaining collars fitted in the bore as intended without any further work.
They do need shortening otherwise they foul the crossover stub.

+1 Thread is same, I used Bronze 750 type and for me I did not need to shorten to clear but as they were bought used they could have been modified already.