pinging on an hill?

I would start by jetting up the secondaries first for the test. If the plugs are THAT clean it could be lean. If secondaries does not resolve it, I would back off timing.

From your combo description, it sounds like a pretty efficient package, but the 10. to 1 compression ratio, with cast iron heads (even if they flow well, they retain heat in cylinders), is pretty agressive on pump gas, not to mention the cam.

Sounds to me like the combo of compression, heads, WOT, and the load of the hill was just a bit much for the pump gas to handle, so it started to pre-ignite, or detonate. If adding a little more fuel doesn't solve the ping, it is now a situation of too much timing for the given load and situation. Back it off 2-3 degrees and you will probably be ok.