Not with an 850. There are some barrel bolts you will not have access too if you bolt the head to the barrel. I suppose it could be done on a 750, but I don't think you'd have enough room to clear the pistons.
I doubt it could be done on a 750 either. However if you must, give it a try. Would need to clear only the cylinder base studs then maybe rock the head/cylinders back since the pistons will pivot on their wrist pins. It will be a major pain putting it back this way.
It is possible to remove the 750 head and barrels whilst still bolted together. There is an article on the fitting a Dunstall 810 kit included in the book "Norton Commando Gold Portfolio" which describes doing just that, along with a photo apparently showing the head and barrels still bolted together being lifted out the frame.
Fitting it that way could be another matter entirely?
I did it on my 71 750 back in 73 when I was having problems. A neighbour helped me because the assy. was so awkward. It is difficult to get the rings back in place as I recall,but I was young and foolish back then. Since then I have had many 750s apart as I restored them and would not recommend doing it that way again. It can be done though.
I have done it twice once with the engine in the frame and once with it on the bench. In both cases the pistons were fitted into the bores with the inner circlips fitted as far into the bores as the gudgeon pin holes and then the whole assembly lowered down to the rods and and pins fitted and outer circlips carefully fitted. If you are doing it in the frame you will need to be strong or have a strong mate sit across the bike holding it all up while you faff about with the gudgeon pins and circlips. Worthwhile way of doing it if you have no need to break the head to barrel joint and all the re-torquing that is required if you do. As someone else said if you have an 850 or 750 with throughbolt barrels it can't be done.