340 converter

Seems as though a cast 340 crank may be a late 72 or 73 which are externally balanced.Check your balancer on the front of the motor see if it say anything.If they put a new convertor in and used one for a 69 340 it may be an internal balanced convertor and you may have an external balanced engine.He may have welded the balancer on 180 degrees around.
My 340 block had no crank and was going in a 318 car so I bought a 318 cast crank for a 70 dart and used the convertor and balancer off the 318.When looking for a crank nearly all the places I checked listed the cast 340 crank as a 318 crank also which are internally balanced.So checking the balancer ont the front will help alot.