I have converted my old bike from 3x9 to 1x11. To do this, I opted for the 11-speed XT cassette from Shimano. This fits without problems on the old freewheel body and the Variant 11-46 offers the right ratio for me in combination with a 32er chainring and 26" wheels in any terrain.
Mounted on cannondale frame with a 34T in front, perfect in conjunction with chain : Shimano HG-701 11-Speed Chain (Ultegra & XT) . Easy to fit.