67 Dart brake change

I have a 67 Dart. The car was originally from L.A. and came with a slant 6. It now has a 340/4speed with 5 on 4 1/2" right hand thread studs on all 4 wheels. The front now has disc brakes and the rear has 10" X 2" drums on an 8 3/4 rear. The brake pedal sometimes goes to the floor when pumped. I filled the reservoir and pumped it several times. There appears to be no leaks at any of the wheels, flexible lines, or hard lines. I'm thinking the problem is that it's the master brake cylinder. But, now with mixed parts, I'm not sure if the master cylinder is the right one (original) or has been replaced when the brakes were changed. The master cylinder has an equal reservoir body and cover which makes me think it's for 4 wheel drum brakes. See picture. There appears to be no equalizer either. Should I have a different master cylinder or an equalizer? And how about a newer style master cylinder? Any suggestions would help. Thanks.

Mastercylinder.jpg