Go ahead and do the duel master cylinder upgrade. Just get a master cylinder for 4 wheel drum brakes. If you are still not sure, get one for a '74 with 4 wheel power drum brakes, that is what you will have. The duel master cylinder isn't there for braking performance, it is there because federal law stated there needed to be redundency in the braking system in the event of a partial failure. You will also need to upgrade the brake light switch at the same time. Stock parts work just fine.
As for "an improvment", what kind of improvment are you looking for? the stock manual 4 wheel drums should be capable of locking all 4 tires if you press hard. If not then you have other braking problems. All the power booster will do is let you brake with less effort from your foot. Any fade, pull, long stopping distance issues you have now will still be there with a booster, maybe even more pronounced with it.