fuel recommendations

Do you know specs of the motor - especially compression ratio (CR)?

If it is stock '68 383 (or rebuilt to) then CR is 10:1. I'd run octane 91. If it's 383 from another year. See here to check CR. MOPAR MUSCLE | 383 CID B Big Block Engine

You ever hear that "pinging" noise (or Diesel engine like noise) in a car especially under acceleration? That is detonation like Skep explained. Too much timing advance and/or too low octane fuel. Essentially the fuel is "exploding/burning" (fast burn) in the cylinder as the piston is rising in the compression stroke. Not good.

Higher octane then more compression needed to ignite. Lower octane then less compression.

You want the fuel to burn when piston is at top of the stroke.

What fuel is in it now? Does it ping?