Cracked 360 Magnum Flexplate

Much mis-information is still out there on 360 Magnum engine balancing.
All 360 Magnums should have the same balance weight. The difference is where the factory placed the actual weight. Some years had weight on the flex plate, some years on the convertor. Some years had a two piece pulley and balancer while later years switched to a one piece integrated pulley and balancer assembly.
Bottom line is if you have a factory Magnum rotating assembly, use the factory balancer or correct weight on the Professional Products balancer, and you get a properly weighted flex plate, you can use a neutral balance convertor. That is exactly how the OP had his set up.
If you cannot find the correct weight flex plate currently being produced, try finding a used B&M number 10242. That should be for Magnum engines running neutral balance convertors.