Wonder if you have the infamous 9" drums. V8 cars are supposed to have 10" drums per literature but some came through with 9"- like my 65. The 10" drums are actually not bad & far superior to the 9" which are best suited for a golf cart.
My '65 and '66 Commando 273-equipped cars both had factory 10" drums. Non-power assist, too, but they did stop pretty well since the cars were relatively lightweight.