Necessary Additives vs. Unnecessary????

Fellow Enthusiasts; I have been out of the muscle car arena for quite some time and have a general concern about additives. When I last had a MOPAR muscle car all gas was leaded (please dont laugh). At a cruise last night the question was asked of me if I use a lead or octane additive in my 340. Just having bought the car this past October, I dont know the total mechanical history of my engine. The engine is very tight and has no leakage what-so-ever yet not knowing if it has hardened valve seats or not makes me ponder. Any input would be greatly appreciated........Don......