What else is required to fit this shredder blade to my FS410C? I have the rider plate, nut and spline washer as I’m currently using the 300mm diameter 3 bladed brush knife.

This would be the correct fixing kit for your machine and would be everything you need: https://www.lsengineers.co.uk/large-metal-blade-fixing-kit-for-stihl-brushcutters.html