flexplate ?

The B&M 10239 flexplate and QuickTime RM-531 bolt-on balance plate are both for LA 360s, which is a different balance than a 5.9 Magnum.
The ATP and Pioneer flexplates are replacement versions of the factory unit. The factory plate (as well as the ATP/Pioneer units, I would imagine) need one of the convertor bolt holes elongated to fit the 904 convertor. Easily done with a round file. Just be sure your convertor is a neutral balance unit (318) and not one with the balance weight from an LA 360.