If you think 9" drums will stop an A-body as fast as a set of disks you're simply ignorant to reality. Let's look at the reality, with sources included.
Here's a road test on a '71 Demon. 10" power drum brakes, and bias ply tires. Well, they had an absolute best stop of 169 feet. That's as equipped for '71, and not even the piss poor 9" drums. 169 feet!!! 9" drums would be worse.
Vintage Road Test: 1971 Dodge Demon 340 – Road Test Magazine Takes A Real Devil For A Spin
Mopar Muscle did some testing on a rear disk conversion on a '73 Dart Sport (a heavier car than a '71 Demon, BTW, but otherwise the same chassis) and checked the stopping distances between the rear drums and rear disks from 60-0.
Their first result , testing just the factory '73+ disk brakes and 10x2.5" BBP rear drums was that from 60-0 the car took 133 feet, 6 inches to stop. Now, this is a radial tire equipped car as well, with bigger tires than a stock Demon, so some of that improvement is definitely tires. But that's already a 36 foot improvement over the lighter 10" drum equipped Demon.
Now, I've heard people here say many times you don't need rear disks at all, drums are fine, and have just as much stopping power. Well, after the rear disk conversion the stoping distance improved to 122 feet 4 inches. More than 11 feet better.
Rear Disc Brakes - All Bound Up - Mopar Muscle Magazine
So no, your drum brakes are not as good, they don't have as much braking power, and they will not stop you just as fast. Even just changing the REAR drums for disks, when the rear brakes only account for 20-30% of the braking, improves the stopping distance significantly. And yes, 11 feet is significant, especially if that's 11 feet into the car in front of you.
And for reference, a boring old 2021 Honda Civic can stop from 60-0 in 112 feet. Which means even if you're NOT tailgating and you're running 9" drums you're gonna clobber that car if it decides to lock 'em up in front of you.
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https://www.carindigo.com/honda/civic/performance
But you don't tailgate. So let's see. At 60 mph your perception/reaction distance is 88 feet. That's the distance you travel before you process seeing those brake lights and actually step on the brakes (and no, you aren't faster than that Rob, I promise). Then, if you're rocking bias plys and drums, well, it's another 169 feet. So that's 257 feet before you stop. How far back do you follow? Cause from the moment you see that Civic lock 'em up they're stopped in 112 feet. You're going another 257. So do you ALWAYS follow at 145 feet back when you're doing 60? I bet you don't. Now, if you're running 15" radials and 4 wheel disks you're only gonna go 210 feet. So that's a 98 ft difference with that Honda. I bet we're all a lot more likely to be 100 ft back from that Civic. Certainly more often then we'll all be at least 145 feet back.
Leave the 9" drums in the last century where they belong.
