Soft brakes after rear disc install

Agent_Orange said
1974 Duster
318 15lbs vacuum
Power brakes
MC front disc rear drum
Factory front rotors
8.8 Ford rotors in back

the car stops but pedal is near the floor before I see any stopping power. Is this a simple MC swap or will I need more gear?

You really need a MC that is for dual disc brakes. Note the reservoir on the OE MC has a larger reservoir on the back for the front discs. There is a reason for that.