Hi DQG,
as with any XS650, there's no room for a single carb between the engine and the frame tube so you'll need to use an asymmetric manifold or get extremely medieval on the frame.
Several aftermarket suppliers sell offset single carb manifolds but frame mods are strictly DIY.
Just about any carb the right size will work if it's jetted properly. We've seen roundslide, flatside and even AMAL carbs reported as being OK.
But if all you want is to be able to run a single throttle cable the simplest way is to swap in a pair of the later model XS650 carbs that are linked together and operated by a single cable.