will 275/60/15 fit 71 Duster ?

Yea, 295 Hoosiers are in but I have a gentle rub on the inside. Will put spacers, but question is what is the safe thickness for spacers with my current studs? Is 1/4" still safe?

With the stock studs even a 1/4" spacer can be too much, it really depends on your rims. The stock studs are pretty short though, so it's hard to get enough thread engagement using any kind of spacer, even with the stock rims.

I used Mr. Gasket #4311 wheel studs on my stock BBP 8 3/4" axles, they pressed in with good engagement. You can cut them down to the length you need after installing them if you don't want the "extra" hanging out.

http://www.summitracing.com/parts/mrg-4311