Motors are ran on test stands all the time, no converter, no problem. Is the new balancer for internal balance engine? External balance will have an extra chunk of metal in the inner ring where internal balance will be smooth all the way around. The only factory original sbc to use external was the 400cid. That's not to say you don't have some weird crank with different requirements. My 385 with Scat crank uses an internal balancer with external flexplate. Good luck.