Vibration Problem

Hello Fellas,, This is the first time I posted a problem so please bear with me. I thought I would ask the Mopar Gurus if anyone could help. I bought the car last fall with a blown head gasket and numerous other "rigged" problems. Anyway I have a vibration starting at 3000 RPM. At first I thought it was around 60 MPH but tried it in park and it is still there. It has a 340 cast only balancer on it and a 10235 flex plate. The problem is the serial numbers on the block are: partial 34 then 0 03028069 thats all I can make out. The best I figured is its a Nov 1968 block. That means it has a forged crank right? Unless the crank was changed, it has the wrong balancer on it. Does this seem right to you all? Im late for church so I have to hurry. Thanks for any help
Bob