Biggest Tires I can put on a stock 1970 Duster

Well you could do a search, where you would find literally DOZENS of threads where this has come up for various wheel and suspension set ups.

In general, a bone stock Duster can fit 275's in the back if you get the backspacing on your wheels right for your car. And that's with the tires inside the wheel wells. Anything larger than 275 requires the ride height to be high enough that the tires and quarters don't come into the same space. So yeah, guys run SS springs and can sometimes run larger than 275's out back, but it's because there's a 2"+ air gap between the tires and quarters and the springs are rock hard. At the stock ride height, or lower, 275 is max without cutting stuff.