I think you need to find another scales and try again.
Are you sure that it weighs in at 3400 lbs.
Yes, it has a Dana 60, but everything else is lighter.
The engine, albeit a Hemi, is all aluminum and has no cast iron exhaust manifolds.
It has (4-wheel) disc brakes, a lightweight K-member, etc, etc, etc.
Do you think that the A/C and Dana add 250 - 300 lbs?
Below are official shipping weights (less A/C) for A-bodies (Dusters):
1970 Plymouth Duster *340 fastback coupe 3,110 lbs
1971 Plymouth Duster 340 fastback coupe 3,140 lbs
1972 Plymouth Duster 340 fastback coupe 3,100 lbs