Locking up the brakes is not really a measure of braking performance, it's like the absolute minimum standard. Very small, inadequate brakes can still lock up the tires with a hard and fast application.
But locking the wheels is not the fastest way to stop, and it is not the most difficult thing for a brake to accomplish. Cars stop faster if the wheels remain rolling, called "threshold braking". Rolling and sliding friction coefficients are not the same. Which means, you put more heat and more energy through the brakes if they're NOT locked up. That's the whole reason ABS braking came about, because the average driver just yards on the pedal and locks the wheels. Which means they don't stop as fast as possible AND lose steering capability at the same time (can't steer a sliding tire).
There are much lighter cars that had bigger brakes than the A-bodies did, even a decade earlier! For example, in 1953 the Austin-Healey 100, which was a smidge under 2,000 lbs dry, came from the factory with 11x1.75" front drums. Those were an upgrade from the prototype, which had 10" brakes that were judged to be inadequate by Donald Healey and his other test drivers. And later even the 11's were upgraded further to 11x2.25's, which is the kind of thing you could only get on a B body Mopar. There are cars that stopped worse than our Mopar's for certain, but there were even 4 wheel drum cars that were MUCH better equipped.
It's great that you've never needed more bracing performance, but both the factory disks and the 11.75" disks will stop the car MUCH faster than 10" drums will. And on a modern freeway you can pretty much bet that most of the cars will have ABS and a stopping distance that's 50+ feet shorter than an A-body even with disks. So for a frequently driven A-body that sees modern driving, freeways etc, 4 wheel drums are a poor choice.
