According to Automobile Catalog, 15 seconds for the Duster 340/4 speed.
Detailed specs review of 1971 Plymouth Duster 340 4-speed offered since September 1970 for North America
Using your website provided,
R/T 2dr Coupe
372 Hp, 401 Lb-Ft., 4041 Weight
5.7 sec, 14.2 @ 100
The V6 isn't that far behind.
SE 2dr Coupe
250 Hp, 250 Lb-Ft., 3719 Weight
7.1 sec, 15.4 @ 90
