Unimog Club UK

Admittedly it says sold out but it appears to be doable.

http://vonsmog.com/unimog_used_parts.html

Near the bottom of the list  406-416 Steering System Complete steering system from a 406. Everything from wheel to pitman arm!. Just what you need to add powersteering to a 404.

Forgive me, I'm trying to learn . . .