If you are running 89-90 octane of alcohol free gasoline the 10:1 compression may be causing a problem. You mentioned having an R Code engine, which could be a 428, 429, or 351 depending on the year, Being stroked out to 408 cid tells me it is a 351, but that could be the 71 Boss 351, or the 1973 351 HO engine. None of those would have had an EGR system from the factory, so I can rule out the EGR being disconnected as the causde of the pinging. There is a way I have been "power timing: engines since the early 70s, taught to me by an ex-racer who became a college level shop instructor - definitely an old school seat-of-the-pants kind of high performance guy. ?before performing this you need to make certain the vacuum advance diaphragm is not leaking or ruptured, that proted vacuum is getting to the vacuum advance diaphragm when the carburetor throttle is opened, and make certain when vacuum is applied off idle (like 2,000 - 2,500 RPM or so) the engine RPM is affected (typically RPM goes up with vacuum applied to the diaphragm, unless you running at an excessing amount of initial timing). In order to test the vacuum advance diaphragm, and test for cvacuum application altering the engine RPM, you will need a vacuum tester.
Although I do not have a video showing how to Power Time an engine, I do show how to use a vacuum tester to test a diaphragm, and how it is used to make sure applied vacuum alters the engine RPM. The video is located on YouTube at:
https://www.youtube.com/watch?v=0yRh_m7TvxE
The parts of particular interest begin at the following video time markers (mm:ss):
01:12 - Two Thermal Port Vacuum Switches identified and described for functionality
05:13 - testing the =Vacuum Advance Diaphragm through the Thermal Vacuum Switch
08:43 - Engine running, I show how going off idle (increasing the engine RPH by opening the throttle blades, Ported Vacuum is created, and shown on one of my vacuum gauges.
https://www.youtube.com/watch?v=ohw1oRlcC4c
07:01 - I use a vacuum tester to apply vacuum to the Distributoir Vacuum Advance Diaphragm to show the engine RPM increases when vacuum is applied to the Vacuum Advance Diaphragm. This proves that the Vacuum Advance Diaphragm is not leaking, and that it is affecting the ignition timing for the engine.
So, now you have been shown how to test to make sure ported vacuum from the carburetor is making it to the Distributor Vacuum Advance Diaphragm, and how the vacuum tester shows the Vacuum Advance Diaphragm is not leaking, and how the Vacuum Advance Diaphragm causes the engine RPM to increase when vacuum is applied to the diaphragm. Once you have verified the Vacuum Advance Diaphragm is getting vacuum off idle, and it is actually affecting the engine RPM, we are ready to set up for the Power Timing. For this procedure we need a tachometer, and a distributor hold-down wrench to loosen and tighten the distributor hold-down clamp at the base of the distributor.
I have not yet produced a video showing how to Power Time an engine, but it is on the agenda for Car Season 2025 for a new flurry of YouTube videos we will be making. I will try to keep things clear and simple in the instructions below. Once you have the tools and equipment needed, and have tested the Vacuum Advance Diaphragm to ensure it has no leaks, and that it is affecting engine performance, you need to make certain the Ported Vacuum Advance hose is properly connected to the Distributor Vacuum Advance Diaphragm. Connect the tachometer to you can watch the engine RPM. Slightly loosen the Distributor Hold0Down Clamp bolt so you can rotate the Distributor a little bit in both clockwise and counter-clockwise directions with a nominal degree of effort - you do not want the clamp to be too loose, as that will allow the ignition timing to drift on its own, never a good idea.
Start the engine and make sure the tachometer is reading. Increase the engine RPM to 2,200 RPM, and hold the RPM steady. While making certain the carbretor is not being opened or closed even a little bit begin to rotate the distributor housing clockwise very slowly. The engine RPM should increase niteable as you move the distribtor in a clockwide direction. Do not move it excessively, a little bit of rotation counter-clockwise is all it takes. As yo are moving counter-clockwise watch the RPM reading. Once the RPM reached its maximum level with the distributor's slow rotation begin to rotate the distributor slowly until the RPM is 200 RPM lower than its prior highest level. Once you have determined when the engine RPM has gone down ny 200 RPM you can let the engine return to idle, han using the Distributor Hold-Down Wrench you can tighten the Distributor Hold-Down Clamp bolt. You to not need to over-tighten that bolt. Tight enough is tight enough.
If you want to see what your iginition time is at for its initial setting you can verify all of the vacuum hoses are properly connected, then using a Timing Light at idle, and the Vacuum Advance Diapnhragm disconencted, look at where the ignition timing marker is locate, and make a note of it.
Now the engine's ignition timing has been Power Timed. It is now time to test drive the vehicle. If, during moderate to heavy acceleration you should not get any engine pinging. If by chance you are getting significant pinging (a little is fine), either you ended up over-advancing your engine's ignition timing, you have built up carbon deposits in the combustion chamber. Use a Bore Scope to look inside the engine to see if there are any carbon deposit(s) in the head's combustion chamber, or your gasoline octane is excessively low. If you have no other issues you should have an engine set to it near optimum initial ignition timing setting. If you have a timing marker you can read clearly I suggest to look at the advance setting in engine degrees, and make a note of it. The setting for ignition timing is now ideal for your elevation, fuel octane level, and engine temperature. If you are still getting some pinging you can retard the ignition timing by one degree at a time until it is either gone or significantly minimized. I would not go less than 6 degrees BTDC.
