I like the idea of trying a higher octane gas before tearing into it.
Palm Beach Race Fuels supposedly keeps the Sunoco product line in stock.
It might be good to just get some 260 GT or GT+ and see how it likes it.
Might even get away with a blend.

E85 would probably be a problem solver as well but have to do more work up front
to make it happen.

Cooling the intake charge is supposed to slightly help reduce octane requirement/ per textbook, looks like the carb is bolted
directly to the manifold? If you have enough room for a plastic spacer, it might help very slightly, but probably not enough to notice.