For drag racing you can put disc brake rotors right on the'75 Duster drum brake knuckles. Just need a caliper bracket to hold the caliper. That combo makes a nice lightweight setup. For street driving it is better to swap in disc knuckles.

The late model drum knuckles are worth some money so don't toss them. I used to use those knuckles for my Viper conversion since they were lightweight but had the large bearings.