Mild 318 pinging

Heat & load are the enemies of detonation. Unfortunately the 'fixes' will require some experimentation. And maybe some $$$$, depending on how far you want to go....
The 2.76 axle is going to load the engine more than 3.XX gears. Heat is the biggest factor; as you found, worse on hot days.
So get the engine running as cool as possible. A smaller w/pump pulley might help. Blocking the exh xovers in the intake should reduce heat soak in the intake manifold.
Cold air induction helps, & gives more power as a bonus.
Then it gets to ign timing. There are quite a few things to try here. You can try reducing the total amount of VA by limiting plunger movement; stronger spring in the dist centri curve mechanism; reduce initial timing 2-4* etc. Final fix might be a combination of the above.