Monolever final drives are getting tough to find. Monolever spoked hubs with remaining brake lining are approaching "hens teeth" status. You can get a 4 lug monolever and mill a later paralever GS hub to fit, but then ideally you'd have to redrill the spoke holes in the hub for a conventional rim instead of BMW's tubeless cross spoke rim.
The dual shock parts are easier to source and you have a greater variety of final drive ratios available to you. Sustained comfortable actual 80mph (not indicated) is a bit high for the 37:11 ratio, especially for 1000cc engine. 32:10 would be about right in my mind.