Somebody has come out with a kit to adapt a Toyota (I think) steering box to a pre-'72 D100. Not sure if any of those parts would help you in your quest or not.

As per RodStRace, I have walked the hot, dusty road of parts procurement, rebuild and install of a D300 power steering (assist, actually) system. It's not alot of fun. Not much benefit compared to a "real" system but it is a big improvement in a big truck.

For my A100, I just suffer along