If your Magnum engine is remaining mostly mild or stock, you can cut the large weight(s) off of your old 727 torque converter and re-use the stock Magnum flex plate. One torque converter bolt hole in the Magnum flex plate will need to be slotted a little to work, so be sure to do that and test fit it before any assembly.
If you are buying a new torque converter anyways, get one that is neutral balanced and do the same stock Magnum flex plate modification.