Discs were standard on '69 - '72 SS Chevelles. The standard 4-wheel drum brakes on '66 - '68 SS Chevelles were horrible and very scary. GM should have never let such high powered cars out of the factory with such inadequate suspensions and brakes. Every BBC Chevelle, not to mention Buick GS, GTO and 442, should have been equipped with front disc brakes and F41 suspension. I'm guessing a lot of lives would have been saved if the company had done so.
I have built and restored many Chevrolets over the past five decades. During the last four decades the first things I have always done involve making the car safer, by rebuilding/upgrading the braking, steering and suspension.