You might be right, here is what you could do.........
Scare up a stock factory 5.9 mag flex plate. This will give you a weight and the rotational and radius position of the weight. cut the weight off and carefully attach it to the flex of a 5.2 flex made for an A500 trans or a 42/44RE (904 based) for a 5.2.
It may or may not be practical to re-drill the 5.9 flex. You could lay one over the other piloted by the center hole of the crank to get the pattern.
