Only weighted flexplates are aftermarket. The usual ones used are B&M. I'm pretty sure they have one for the Magnum application, but I don't have the # handy.
I helped a friend install a 5.9 magnum in place of a LA 318 in his '74 D100. His 5.9 had a factory weighted flex plate and needed one of the holes opened up a bit. We unded up using the entire harness and electronics from a 94-95 ram and keeping the EFI.
