If the block states its a 72 340 then it is out of a 73 car. That would probably mean its a cast iron crank. If you don't have the original torque converter in the car or balanced for a cast iron crank you will get a bad vibration.
Many times I have seen this issue because of that problem. Something you may want to check. Look at the Damper and see if it has the words cast crank on it behind the crank pulley. That could be your issue. Just had a 360 car here with the same vibration. Do you feel a vibration at steady higher RPM's in neutral.