Well, since I am over 50 i have an excuse when it.comes to my memory, ha ha. Anyway, if I remember right someone here on fabo had done a conversion from single circuit to dual circuit brakes and used the master cylinder from 1967 or 68. But, I think it said that a master cylinder from 1971 or 72 or so would have been a better choice, it would have required less push with the foot to stop the car.
A few things to consider, not knowing the bore of your single master, i will go with the 1.25” one in my fargo. When switching to dual you will want to go to a smaller bore. Dual masters displace 2x roughly what single lines do.
The common one used in the conversions is 15/16” bore.
That matches pedal travel and feel.
Im considering trying the 3/4” one i have on a power brake setup from a pt cruiser. Its disc /drum but i think an add on residual valve on the disc part (front) will solve excessive pedal travel.
Disc brakes are 3-5 psi where drums are 10. Someone please correct me if im wrong.
Higher psi holds against return springs to reduce full return of shoes thus reducing pedal travel.
Disc brake calipers require less pressure to keep pads from moving away from rotor.