There is a bzillion cars running around with the 9" drums and hasn't been an issue so far. I wouldn't worry about it unless someone was going to do some road racing or something.
I don't think there ever was a "bzillion" cars with 9" brakes, there certainly isn't today. That not with standing, the issue is; back when these cars were new everyone's brakes stunk so if the guy in front of you made a panic stop from 60 mph it took him the same 200 ft as it did you to stop. Today that same guy in front of you will be able to stop in 120 ft or less and it will still take you 200, that my friend is a problem. With all the conjestion on the roads even adding an extra 80 ft of space doesn't work because some one will fill it up as soon as you back off. That's not to mention all the accidents that occured with immoveable objects that would have been completely avoided with today's braking systems.
I speak from experience the 9" drum brakes in A-bodies in a panic stop from 65 mph will slow you to about 45 then heat up and fade to the point that they no longer slow the car and you have to lift your foot to allow them to cool before you get any stopping ability back. Even at 45 mph your heart will be in your mouth if everyone in front of you is grinding to a halt and you are waiting for the brakes to cool. You likely have heard the saying "my life flashed before my eyes".
With all the conjestion on the roads these days it is mandatory (IMHO) that you have a braking system that is on par with the cars around you. If you have 9" brakes I would recommend that you upgrade before even registering the car.
The factory disk brake systems from 40 years ago coupled with the modern brake pad materials available today will put you on par with 90% of the cars on the road.
