Use a master cylinder from a later car with the same brake setup - drum/drum, etc.
You will need to be able to do double flares on the brakelines. A little practice would be good.
If you want to replace all the brake lines, now's a good time! Kits are available.
At the distribution block, disconnect the rear brakeline. Cap off that port on the block. Adapt the old line or make a new line to go to the rear of the master cylinder for the front brakes.
Make a new line to join the rear brake line through a union to the front of the master cylinder, paralleling the original line.
Oh, and remember to put the fittings on BEFORE you bend and flare the lines.
Flush and bleed. Check for leaks. Bleed again if needed. Check again.
