1973 duster 340 counter or neutral balance

Look at the harmonic balancer, it will have a counter weight on it and I think it will say "340 cast crank only" too. Note that you need a specific flywheel for 340 cast crank engine. (it is different fro a 360 and a 360magnum) The old MP chassis manual tells you how to modify a neutral balance flywheel to work.