The problem is that it depends on the metallurgy used in the specific engine, its current condition, and how it's driven so there is no one answer. The majority of cars that old that have survived are weekend toys, not daily drivers, so it would take a long time for problems to develop.

You would not want to add actual tetraethyl lead as it is extremely toxic to handle. However there are substitutes available that purport to cushion the valves in engines that were designed for leaded fuels.

Now if you were planning to rebuild the engine down the line anyway I would say just run it and when rebuild time comes install hardened valve seats and a roller cam and be done with it. However if you just want the engine to run as long as possible without tearing it down, the high-zddp oils and additives as well as the lead substitutes are readily available so it probably makes sense to use them. The additional cost is negligible compared to the cost of a proper engine rebuild.

I've been running E10 gas in my '71 Ambassador and '67 Rambler with no apparent problems. (There are no gas stations in the area selling non-adulterated gasoline.) However rubber fuel system parts have been replaced with modern materials. This is something that should be done in any event when you're talking about cars whose age is measured in decades. So depending on how old they are you may need to replace rubber fuel lines, fuel pump, and rebuild the carburetor. (You said the carb was original but it's probably been overhauled at some point over the last half-century.) If you can find pure gasoline though that would be best.

The need for lead and zinc additives is somewhat controversial. The general consensus seems to be that on a broken-in engine if you don't have a high-performance setup with high-rate valve springs or aggressive cam, or you're not subjecting the vehicle to heavy duty use like trailer towing or extended high-speed runs, they are not needed. There are of course high-zinc oils and additives available if you want to be on the safe side. (Do an internet search for "high zddp oils"). Also lead additives and substitutes.

I haven't done anything special and have not run into any problems but I'm not running high-performance engines and those cars are used very little.

I recently bought a 1970 Mercury Marquis with a 429 V8 in it (original carb although it probably has been rebuilt). It runs fine on premium E10 gas. But, I've heard that ethanol isn't exactly great for some rubber parts in the fuel system and inside the carb (and a fiberglass fuel tank if I had one). I'm also not sure about adding lead additives to the tank. Some people say that if I don't add any the valve seats will wash away, but some say that with it's age (and a possibility of already being run on unleaded for years) this really shouldn't matter.

I'm not sure what side of the story to believe so I decided to bring up these questions.

Question 1

Will ethanol be a problem? If it is then what needs to be replaced?

Question 2

Do I need to add lead additives every time I fill up? And If I do, what would happen if I don't?

If it matters, I am planning on using this as my main vehicle.

Many thanks!  😉