Assuming your 850 currently has dual carbs, I'd go with a new set of Premiers. You've already got all the hardware, manifolds, air cleaner, cables, splitters. You can buy a set of Premiers for $350, bolt them on on and your 850 will be running again like God intended.
Easy-peasy.
With a single Amal you'll also have to buy a 2-1 manifold, cables, air cleaner, and you'll probably have to experiment a bit with jetting if you run a single Amal. Cost will be a wash.
I know a lot of guys run single Mikuni's and they really like them. Haven't heard many people with a single Amal on an 850.
I had a single Mikuni on my 850 and it's performance was flaccid (I know, apples and oranges). Yeah, it's a lot easier not having to sync dual carbs. But it's worth the effort in my humble opinion.
Just my $.02.