I have a solution for this, I had exactly the same problem with a 360 in my 70 Cuda. You can find the thread if you click my username.
Put the flywheel up in a wheel balancing machine and you will find out how much and where it has been balanced. Weld the same weight on the opposite side (180 degrees) on your zero balanced converter and you will be fine.
It is important that you have about the same distance from the center, so measure the converter first so you know what distance you should use when you put the weights on when you zero balance your flywheel. Good luck
Wow - hope you understand!