5.9 Magnum to TF904

I don't have much experience with the magnum motors and the flexplates but I did have to swap a trans in our old 98 ram 360 4x4, the new trans was from a 318 truck and I think I remember finding out that the flexplates on those have to stay with the converter, not with the motor to balance that. Also wouldn't putting a 518 converter into a 904 create an issue because the 518 uses a lock up converter?? Hope this helpes someone