Will this work on a 1976 Dart sport with power disc brakes in the front?

Looks like a 4 wheel drum master cylinder, so no.
fyi, several mfgr's sell masters that look like this that are disc/drum as well as disc/disc
here's one from "right stuff" Right Stuff Detailing DBMC04 Right Stuff Detailing Master Cylinders | Summit Racing
here's another from CPP Classic Performance M-C118-L Classic Performance Master Cylinders | Summit Racing