Agree with your measuring system - when I lay a ruler along something I see which system fits the closest or is the easiest to read and usually end up with inches along the length and mm accross the width. I could easily have a box 6.2" x 16mm x 3-1/8"
So to get DC from that set up you'd need a diode on both the 50 and 80 volt lines as well as the return to get selectable 50 or 80 volts DC.
In addition another choke past the diodes to smooth out the ripple.
I guess you could use a 4 diode bridge to get smoother DC to start with.