Thought I'd post an update on this. I stocked up on some idle and air corrector jets, and started experimenting.
I wound up with a 070 primary idle jet (one step richer than originally installed 065), and that seems to have completely eliminated the part-throttle pinging I was hearing.
The primary air corrector jetting was interesting. My two carbs weren't matched originally, one having a 150 and the other having 155. I started out with a pair of 155's, which is the leaner of the 2 sizes. It ran fine, but didn't seem to pull as well. So I switched to the pair of 150's, and it feels much better.
So the bottom line is that I'm now running a richer primary idle jet, which affects the bottom end primary-only running, and I'm running a slightly richer primary air corrector jet package, which affects the top-end of the primary only running until the progressive linkage starts to open up the secondaries. I haven't changed anything in the secondary jetting.
Now I'm starting to wonder if the heating up at freeway cruising speed and the detonation heard as it was starting to heat up were really indications of the primaries being too lean and not issues of too much ignition advance. So my next step will be to remove the tiewrap that is currently locking the centrifugal advance on my HEI and see what happens next.
Keep in mind that I am running an overbored 230 running 9 to 1 compression with 91 octane gas, cleaned up ports, Edmunds intake manifold and head, a mildly upgraded cam, an HEI ignition system and cast iron headers into a low restriction dual exhaust. The changes I have made may not be appropriate or necessary for a more stock engine.
Marty