Converting from single master cylinder to duel ?????

If it was me I would put in a proportioning valve from a newer 4 wheel drum brake car, I did with the 66.
A proportioning valve helps if one of the front or rear line ruptures you still have brakes. The same is true if you just run a line out of the master cylinder to the front and then a separate to the rear, but then you have to use brake line T's for the front lines