B&M 10242 (5.9 magnum) Flexplate shown with 904, 727 and magnum flexplates. I see no reason why the 10242 would not work with the 904 even though it is listed for 727 applications.

NOTE- Not all magnum flexplates are the same! Early units had the 5.9 weight attached to the converter, later ones were attached to the flexplate.