Like Eric said, the single circuit MC won't work on a combo disk/drum setup. If you leave the residual pressure valve in, your fronts will drag. If you take the RPV out, you'll have to pump the pedal to stop as the rear brake springs will collapse the slave cylinders all the way in.
I see you live in Redding, CA. There is a Pick-N-Pull there. Find a 1979-1989 Toyota minitruck (2WD or 4WD) and get the master cylinder. It should cost you about $12 without brake booster or $20 with booster. You will have to redrill the firewall to make it fit, but the bolt pattern is the same whether you use a minitruck or a cruiser master and booster. The minitruck master is already valved correctly for a front disk / rear drum setup.
Dave