A518 Flex Plate Help???

The 3 wire indicates an RH but The symetrical pattern converter is a clue that its 97/newer. -46RE-
The RE converter would also be unweighted/neutral.
Meaning you will need a balanced flexplate for the LA 360 or weld on weights to the new converter.
That won't matter tho unless you find a stand alone computer to run the RE.

Yes, balance is a factor in either case. the Magnum and LA are different weights.
You can use the B/M balanced flex plate for the LA, and knock the weights off the converter to make it neutral.
Or get the 360LA weight kit and change the balance on the converter.
But not on a symetrical bolt pattern, cause you have a 75% chance of bolting the converter to the flexplate in the wrong position.
Thats the whole reason for the one lug being offset. -no mistake possible-