Again: Unleaded or leaded fuel for mid 70's /6?

If I’m not mistaken, the higher the octane, the slower the fuel burns and less prone to early detonation from higher compression.

As for the OP’s question, if the engine has not been rebuild and had hardened seats added during the top end rebuild, use a lead additive and pump gas. Leaded pump gas is a thing of the past. If hardened seats have been added, don’t worry about the lead additive. If you can find “super clear” aka non ethanol gas, run that.