Use a stethoscope and poke around on the different engine components while it is running and see which area is the loudest of the knock.
Even check the transmission areas too while running, that sound travels through everything.
Distributor, intake, valve covers independently, timing chain cover, fuel pump, oil filter area, block, oil pan.
Infra red heat temp gun can check the exhaust temps for each cylinder at the header tubes once the engine warms up. If you have a cold cylinder that could lead you to your problem.
Especially if one of your cam lobes started going flat. Need to use the Zinc additives for the high lift hydraulic lifter cams of this era. Today's oils don't have the Zinc, and cams can go flat, especially in a higher performance setup.
