Nothing simple...
I have installed Disc brakes on my 6400, but they came with newer axles, the rear is simple, the front takes some work. It is not recommended to have disc on the rear without the front frist.
My method allows you to keep the original bolt pattern front and rear (5 and 10 on 7.25" circle), the center hole is larger, but new wheels are available, or you can have your old 20" ones bored out to fit.
Check out my pictures on the link below.
Grigg