The years of the engines could be a problem, in some years the 273 crankshaft has a smaller register for the converter pilot hub, to mate the 273 converter to a later LA or Magnum a spacer ring is needed.

If the Magnum is '96 or later and the crank position sensor is going to be retained the original flexplate must be retained and redrilled for the 904 converter.

Years make a difference, it's best to be specific.