I agree, stock 340's would be around 14.5 - 14.7 on the tires available back in day. A well tuned piece with a 4 speed and a good driver could do a little better.
I ran a '89 D150 truck with 360 tbi, single exhaust, 3.21 gears at the track. It ran 15.3's. I wonder what it would have done in a much lighter Duster cutting less wind with dual exhaust, a perfect working TQ and 3.55's.