Well, Ballard does have a dimensioned drawing rather than my sketch, which I see you still have, as an easy description rather than a thousand words.

Still, some/most of it was empirical by trial fit, eg:

The front face of the disc to the shoulder ~ 1/2" to allow reasonable clearance of the viscous unit mounting bolt heads to the pulley bolt heads.

The larger diameter of the shaft was made the same as the viscous bush outer diameter, whatever that was.

We used a 3/4" BSF nut (because we had one) with self made castellations and turned the shoulder to match the viscous bush.

The thread was made to be just 1/2 a turn too much and sawn off to show two threads through the nut.

The shaft was machined until a moderate press fit in the viscous bush.

The split pin hole was marked and drilled after the nut tightened.

New and longer 1/4" UNC bolts were hand tweaked for exact length in the pulley bosses to utilise the maximum thread area.

A real selective, one-off, assembly really.

R.