Had that problem with a '69 340 in a buddy's car in the early 1990s. I had a fresh small block dual point on the shelf so we put that in. Car ran perfectly so I sent him home with my distributor. I went through his which included cleaning hard grease out of the advance plate bearing. Everything seemed perfect, but it still was pinging. Went out to a local yard that still had a bunch of factory dual points in search of a better breaker cam. After replacing that all was well with his car. I didn't have a dwell meter at the time, which may have shown the issue, but that is very likely the problem you are having.