just completed a rear diff rebuild on my syncro.
Every thing came apart fairly easily until I came to the 41mm nut on the input shaft. VW have a special tool which fits on the viscous spline and then use a ring spanner on the nut.
Not having either I looked into the option of a 41mm deep socket but it doesn’t over come the problem of locking the shaft.
Having access to a sheet metaL laser cutter I drew the spline the nut and the housing on auto cad and had various plates made which could be assembled together to make my own nut locking device and a boss to mesh over the splines allowing me to turn the shaft whilst keeping the nut still. Worked a treat without any unnecessary forces.