identify cast or forged

could be a number of issues. wrong torque convertor, wrong balance on drive shaft, wrong balancer, wrong crank, wrong flex plate.
I once bought a valiant with a 360motor and 318 trans. took my a while o figure out what was out of whack, but in the long run it was a cheap fix by buying a balance weight kit and welding the torque convertor.
take a picture of the balancer and post it to start, then remove trans inspection cover and inspect for balance weights on the torque convertor. also look on the side of the block for a casting date.
This is some place to start for now report back what you find and post pix if you can.