Your friend is confused. The LA engine in your Dart is indeed lighter than an A engine. However ALL 273's, 340's, 360's and 67-up 318's are LA engines. The A engine was the polyspheric head engine, which dates to the late 50's, and was an economy outgrowth of the early Hemi.
There was a T/A version of the 340, but it was slightly heavier, and never installed in a Dart.