booster addition to manual brakes

No reason for a booster w/ drum brakes. The design makes the shoes "self-jam" against the drum, which greatly reduces pedal effort. As mentioned, a smaller bore MC would make the pedal easier, for slightly more travel. I would first refresh the brakes. Drums are much more sensitive to gunk on the liners.

Don't believe all the claims that drums can't stop the car. If they can skid the tires, you can't do any more braking than that. Drums will overheat easier, like if one (stupidly) rides the brakes downhill (reason for U.S. mandate for front disks ~1973). They should not overheat and fade from an occasional stop from 70 mph. Might be marginal if braking from 100 mph.