I have a 72 340 matched to a 904 automatic trans. I am getting an engine vibration at and above about 1500 RPMs. A friend was telling me I may have the wrong torque converter for this engine. Anybody have any info or insight on this? Not sure if it is a steel or cast crank, I have also been told they take different harmonic balancers. Is there any diagnostics I can do to figure this out?

first find out what (balance related) parts you are running. What dampener (in or ex balance) up front. If the TQ does or does not have weights on the front flat face of it. Whether the eng is in or ex balanced (someone knowledgeable on 340's will help you with that). did the eng run OK in another vehicle?

Don't anything about the motor, Just purchased the car and once I got the miss out of it,( vacuum related), I noticed the vibration. Is it it supposed to be a weighted TQ if the motor is externally balanced?