They did change on the magnums about the same time as OBD2 came out, the "up to" OBD2 were not weighted, the weight was still on the converter, and still had 1 hole slightly off of the 4
that held the converter to the flex plate
The "after" (about 96 and up) there were no weights on the converter, 3.9/318 and 360 could have the same converter/ and there was no more offset hole of those 4 bolts.
The face of the converter was marked (stamped) "90*" meant that all 4 bolts were equal spaced at 90* apart. Either setup will work as long as you have either all "before" or all "after" converter and flex plate.