Yes, AVGAS 100LL is a leaded gasoline for aviation use only (and you won’t find it at an auto gas pump in any State). If you can get it from a general aviation airfield, it will work much better than just unleaded gas. I have a '73, which is virtually identical to your '74, and after trying to retard the timing a bit using premium fuel with some success (reduced pinging; some with run-on when hot and poorer acceleration performance), I ultimately went the route of a “no-lead” head conversion (stainless steel valves and stellite seats, head planed flat, as well as a copper performance head gasket) and continue to use premium unleaded fuel for the best results. No run-on or pinging and much better performance since. Well worth the cost.
I am also adding an electric fan this winter to keep the engine temp down, which can also help reduce pinging and potential valve damage.
If you use the car on only a limited basis, have problem with pinging and prefer not to spend the time and money on revising the head, premium fuel with the timing retarded 2 degrees or so generally works. The auxiliary electric cooling fan can help as well.
I hope that helps you.
Best regards,
Art Isaacs