74 dart vibration problem.

after a little research, i belive all 318's were produced internally balanced... however, if someone put a 340 crank in that was from a later 340 it would need a correct balancer.

it could be that you have a 360 balancer on the front.
they look different. this page has pictures of the 318/steel340 as well as two of the 360 balancers
no picture of the 340 cast crank balancer
http://www.bouchillonperformance.com/BPEHarmonicBalancers.asp