Can I fit a 345/50-14 tire on a stock 71 duster?

I think you missed it by that much.
Since your stock rear house sticks out an extra two inches,per side, you will need the same extra 2 on the backspace, and then subtract 1 inch for the narrower rims, so by my math, you would need 8.25bs. But wait.
You still have to move the springs.
And, notice he said, to get the tread flat on twelves, he is running 25psi, Bias plys are not like radials. You can run 295 radials on 10s@ 25psi, but if you try to run those Ns on 10s@25, the edges will lift off the road. If you run less than 25, it starts to corner like crap. The body rolls over until the sidewalls, and say "no more". Then the inboard treads are flat to the road, while the outboard treads are lifted up. And maybe the sidewalls get into trouble with the body or the springs. So I have been running 24psi for 10 or more years, in the 295Radials,on tens.I have gotten used to the way it corners, and it hooks ok @24 and the tire-wear is minimal. I tried running a little less, but went back to 24 pretty much right away.
I'm kinda thinking the Ns absolutely need 12s