Disc brakes became standard in 1969 for the SS396. There are lots of copies of the 68 brochure online. I always like to look at them when I am doing a near stock build.
I always heard the same thing about special paint on GMs back then, except that the selling district office set the price. I also heard that Cadillac Firemist colors weren't available because the larger flake wouldn't work with the paint equipment in the other assembly plants. Could be true, could be internet BS. I did look a "Resale Red" 69 SS396 convertible that had a blank paint code. The seller said it was blue under the carpet, etc. I have always wondered what it looked like new and why the original buyer couldn't find a factory blue they liked.