It’s likely rattling right around peak torque with a higher load. Sounds like you have the timing scienced out pretty well, but I’d recommend experimenting with slowing the curve down a few hundred rpm and delaying it a couple degrees through the middle. Keep the big timing numbers at cruise for mpg. If you have an adjustable v can, adjust it to come in a little later. Once the engine spins up a little does the pinging go away? You might also try a slightly colder thermostat, plugs, and/or a little more fuel if you can.
