You don't need a lead additive.
Your engine will run just fine on unleaded fuel with no ill effects.
I've never owned a modern car and they all run unleaded with no additives.
Contrary to popular myth, unleaded gasoline has been around since 1923 when AMOCO introduced it's famous unleaded "white gas" super premium.
Your biggest worry should be ethanol fuel. If you drive your '53 all the time and use up the fuel in the tank in a timely manner, there won't be a problem. If your truck sits around waiting for the next car show, only fill it with ethanol-free fuel. All of this assumes your carburetor and fuel pump have been rebuilt with modern ethanol resistant parts.
