A little more than that mate.
Yes a proportioning valve is the way to go.
Another way is to play with the size of the slave cylinders.
The thing you need to worry about is you need a one way check valve fitted to the master cylinders rear circuit.
What happens with drum brakes is that after you have applied the brakes and released them, the return springs pull the shoes back inwards.
A one way check valve allows the shoes to return a little and then stop returning, which keeps the shoes near the drums.
Without one what happens is your shoes would be returned all the way back till the slave cylinder pistons reached their inner limits.
Then when you pressed the brakes, the shoes may not touch the drums, or the pedal may have a lot of travel or need to be pumped.
Take it into a brake specialist, they may be able to sell you a one way valve to the rear circuit or both front and back if you have 4 wheel drums.
Other wise just buy a master from a car with rear drums.
The one way valves normally sit under the point when the brake pipe bolts onto the master, sometimes there is a large nut with the hole and thread for the brake pipe to bolt into, removing it exposes the valve, all depends on the model of master cylinder.