Couple of questions ...

Cuda416, you have the flexplate info correct. You need 5.9 Magnum-weighted flex plate so you can use your neutral balanced torque convertor. The factory flex plate from the donor truck can be used by slotting one convertor bolt whole when running a 727 trans, but I don't know how bad the mismatch is for the 904, and I don't have a 904 convertor laying around to try it on. I think your 904 has a smaller diameter bolt pattern.