While I can't say for sure if the LA flexplate would work on a carb-coverted Magnum 5.2, I do know for sure that the Magnum 5.2 requires the factory flexplate to run its stock MPFI, as the crankshaft position sensor takes its reading from it (see: http://www.dakotaforumz.com/showthread.php?t=2753).
I'm fairly sure the bolt circle's the same on the crank. The TC pattern's not the same- for a Mag flexplate it will need a bit of oblonging to use the stock TC. You can use the stock MPI flexplate even if you don't run MPI.