My brothers and I learned to drive in an old 1957 3-speed slant-six Plymouth. While we burned out the clutch in the Plymouth in 1966, my mother bought a 1964 Ford Custom (cheap, with no body trim) that had been a NC police car (for speed-traps and to catch bootleggers). That very plain looking 4-door had the full Police Interceptor package (with a special 390 Thunderbird engine). The early Little GTOs (3-deuces and a four-speed) were no match to this special 4-barrel and a police automatic transmission (that shifted into high gear at 80 mph).

Back to leaded gas: That 390 high-compression engines would now need leaded gas or hardened valve seats. As mentioned above, the 216/235/261 engines are probably OK with modern gas unless the engines are run for long continuous periods at very high RPM.