I'd say so. I haven't really dived into the 71 models. On the 66s, the fitment rate of disk brakes on the 273 HP cars was somewhere around 12%. Given the way the cars were marketed, I'd think a disk brake /6 would be a rare bird prior to 73.
The only cars I've dependably seen 8 3/4 under were V-8 4-speed cars. Once the BB & 340 showed up, they got 8 3/4ss. The 273 HP 904 I owned had a 7ΒΌ rear. FWIW I've seen /6s with the Taxi package that had 727 trannys.