I had a wrong t-converter on a 340 engine once, (steel crank, cast crank converter) and the engine ran great but my friend noticed that the crank would walk out or move forward when the engine was reved up. It did have a vibration but wasn't sure what it was. Well, found out later that the guy who installed my converter thought the engine was a 74 cause the car was...the engine is a 69....my dumb fault for not knowing and his dumb fault for not checking...oh well. Anyway, my thrust bearing (center main) had worn out from this, the bearing actually wore into the sides of the crank weights, causing the crank to move like that. Had to pull it out, damn, lesson learned. I had the crank fixed and it is holding up great but was a spendy mistake. My engine also had the right balancer on it, I would pull the pan and check for play or maybe pull the center thrust bearing and check it? All my other main and rod bearings were fine but if I remember right I replaced them all to be safe, the engine was only rebuilt about 1-2 years old at the time.
