I think you are headed in the right direction. Yeah, you definitely need to get the centrifugal advance dialed in before the vacuum advance. Then you can measure the cruise vacuum and tune the vacuum advance so it only operates while cruising and drops out when you accelerate.
The 389 centreplate with 139 weights gives a total of 21 deg advance according to Rocky Rotella's article in the May 2004 HPP, which matches your test results. Your cam is pretty mild so you can't get too aggressive with the timing. Normally you don't want advance starting until about 1000 rpm so you must have quite weak springs since it starts at ~600 rpm. Stronger springs will slow down the advance.
Running hot or a lean carb setup will also cause pinging, and those have to be fixed first before the timing can be optimized.
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