A lot of 340 Dusters came with 3:23's from the factory. I have an 8 1/4 with 3:55's behind a pretty warmed up 318, so far so good. My engine builder told me my rear-end, built right, could handle 400- 425hp. I do believe my next upgrade may be a 8 3/4 rear. Also, I love the look of your Duster...