It is useful to know which of the eight cylinders is detonating.

The inexpensive 'Fires in Oil' sparkplug extension sleeves can be used to find which cylinder is the most prone to pinging.
The extension sleeve has an effect like retarding the ignition timing in that particular cylinder,
plus it adds a couple cc's to the combustion chamber, which slightly drops compression ratio.

Another trick to narrow down the source of detonation is to temporarily use 2 cycle synthetic oil.
This more expensive 'ashless' oil has a much better 'blending' octane rating that 4 cycle oil,
and if the pinging goes away on a test drive with the 2 cycle oil in there, it indicates an oil control problem in one or more cylinders.

Since you are still using a conventional distributor and cap,
once you find the troublesome cylinder(s) it is possible to modify the interior posts on the cylinder cap to retard the ignition timing cylinder by cylinder.

As you approach the 'ideal' ignition timing for a cylinder's performance,
called the 'Mean Best Torque' or MBT timing,
you will find that there is about a six degree span over which torque does not increase or decline... kind of a flat plateau instead of a sharp peak.
The timing needs to be on the 'retarded' edge of this plateau, because unlike torque not changing,
detonation likelihood increases even faster with ignition advance as you approach the limit.