360 engine vibration at ~3000rpm

Need some more info
What Harmonic balancer , the one with a cut out or the weight
What flex plate do you have , one with a weight or the one with the cut out
What about the torque converter , does it have a weight or two or nothing
Do you have poly engine or transmission mounts
What is your timing at initial and total, I know you said it is back but just covering ideas.
Was this engine previously balanced , any idea about the short block
Do you have pics of the parts that came out and then installed
I know there are several questions , just trying to narrow everything down.