the distributor itself doesn't do much like it did on older 318s. No advance can, no advance weights, unless the bushings went bad (I havent seen one off a Magnum engine yet with bad distributor bushings) replacing the distributor itself would be an almost certain waste.
what brands of parts did you use? I had a new cap on a 3.9 in a Dakota recently go bad on me in less than 5k miles. Napa Echlin too. and quantity of miles on wires doesn't speak much for whether they could have a problem.