My 1977 TY175  (new to me) clutch engages very late and is hard to control in a section. The cable is lubed, the actuator cam is new and greased, fibre plates new with flat metal plates. 

Do I need to lengthen the actuator arm? What am I missing? Help is appreciated. I am looking for a smooth controllable clutch engagement as opposed to an off/on switch.

Wear grooves in the basket fingers can cause sudden engagement. You didn't mention removing the grooves.

Where the lever is when the clutch engages is adjustable via the cam and cable.

First set the cam to give 90 degrees angle between cable and cam arm at the point when the clutch starts to disengage. If you can't get 90 degrees then there is something wrong.

Set the cable adjustments to put the release point where you want it. If there is not enough cable adjustment then there is something wrong.

If I remember correctly there is a screw and a jam nut that moves the shaft which pushes the clutch push rod  up or down this allows different amounts of clutch rod push movement. This means quicker or slower clutch release and also engagement.