70 Duster front drum to disc conv.

I haven't done the swap yet myself, but I'm pretty sure (someone correct me if I'm wrong) that discs weren't mandatory on your car, but were an extra cost option. Meaning your car most likely came with drums in the first place.
Secondly, if you want to keep the 5x4 bolt pattern, you will have to find a donor car with discs with the small bolt pattern. I believe all aftermarket kits are the larger bolt pattern. But again, someone correct me if I'm wrong.