Very well could be. The AMA specs list the curb weight for a 1974 Scamp with a 318 at 3190#. That's 205# lighter than the '74 Duster 360. Now, the Duster came with HD suspension (+20?) so maybe 180# more?
Add the 120# difference based on MY and you are at potentially a 300# difference between a 1970 Dart 340 and a 1974 Duster 340.
BTW, this makes it look like a 1974 Dart Sport 360 was 15# heavier than a 1974 Duster 360?
Didn't realize there was that much difference between a Dart and a Duster. Doesn't feel like an apples to apples comparison to me though if looking at the 360 vs 340 alone.
Not saying the 360 is all that either. I have what I have and it isn't stock so I don't really care how fast they were when new.