Plymouth launched the Duster as a 1970 model — A-body Valiant chassis with a fastback roof line, marketed as a youth-oriented economical sport coupe. The Duster outsold the Dodge Demon by a wide margin (1971 Duster volume was roughly 173,000 vs Demon 60,000) and continued strong through 1976.
Engine lineup followed the A-body convention: 198 / 225 Slant Six, 318 small-block, 340 four-barrel for the high-perf Duster 340 trim. The 1971 Duster 340 was a popular budget muscle pick — performance on a smaller, lighter chassis than the B-body Road Runner.
1973+ the high-perf small-block changed: the 340 was replaced by the 360 four-barrel for 1974. Plenum's Duster 340 trim fades from the catalog accordingly.