booster addition to manual brakes

AJ/FormS said

As to those 9 inchers; they were designed for 13 inch tires, which are very easy to overpower. If you upsize your wheels and tires, there is a whole world of difference in stopping power available to you, with bigger brakes.
The spindles that go with those 9 inchers may not be the strongest either. Back in the day, they were known to break under heavy loading.It even happened to me. I will never run those on anything I drive. I have no idea how common it was. ...

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Perhaps with sticky drag tires w/ a large OD (jacked up rear), 9" drums wouldn't be able to skid the tires. Depends on how hard you can press the pedal. Not sure what "stopping power" means. If power refers to "energy", the same heat energy will go into the brake drum regardless of the tires. The energy is simply the kinetic energy of the vehicle: E = 0.5*mass * (velocity)^2.

I have read reports of 9" spindles failing. It is forged metal, so hard to imagine what could cause that. Perhaps if you hit a high curb hard, it could break the goose-neck, but that might not be a bad thing (prevents other damage). The weak point I know of is the smaller wheel bearings. The outer bearing spun on both sides in my 69 Dart. I think most slant six engine cars came w/ 9" drums (thru 1973), but never say "all" w/ Mopar.