I've also owned a couple of 340 dusters. Had a '72 Duster 340 (it had 360 heads on it because I had the "J" heads on a 360 I was building). The 340 did have a slight upgraded cam (.474 lift, 280 advertised duration) and headers, 3.55 gears. All else stock, including tranny and converter. No slicks, and traction was terrible at the strip (2.3 60 ft times). The car still went a best of 13.59 at 104 mph. Slick would have dropped the 60 ft times to approx. 2.0-1.9 range, resulting in about a 13 flat to 12.90 range.
