1971 Dart (Swinger) was not available with a 340 unless you lived in Canada and ordered from one of two dealers. Crestview or Crosstown only. They were a limited run special order from Chrysler Canada and not available in normal production or in the U.S. no matter what codes you used or who you knew.
There were also a very limited number of Canadian only 1972 Dart Swinger 340 cars built as well, possibly 40 or so. Pembina Dodge in Winnipeg also sold a few of them in 71.