A 70 Duster had a 7-1/4 or an 8-3/4. You need to know what he has first. The cheapest alternative is a junkyard 8-1/4 that is already big bolt, but watch the gear ratios. The one I pulled for somebody else was a 2.45:1 (no, that's not a typo). I don't like the redrilling for several reasons, but the big one is that now you have brakes that are already hard to get drums for, but if you need them you'll have to get them drilled out, too.