OP - so now comes the unwanted English lesson. What it says is low speed AND under a load, not low speed OR under a load. They mean both things at the same time.
Thought #1 - Are you trying to made this car exactly like it came from the factory? If not (and I know there will be some howling here) just replace the system with a factory electronic ignition. If it's a 72 it would have come with it if it was built later in 72. You can go to @halifaxhops (another member, who builds Mopar distributors all the time) and buy everything you need to convert it and, if you answer all his questions, he will deliver a distributor that you know has an advance curve that fits your car. This will save a HUGE amount of time and money because you don't have the equipment and knowledge to fix this. You have to get rid of detonation because it will eat your engine. I'm already worried because you have driven it like this through 3 tanks of gas!!!
Thought #2 - yeah send the video because if it's not detonation it would be great for you to fix whatever is wrong.