Detonation ping

[kwmech]

The intake manifolds in those years had a tendency to crack through the EGR system and create an internal vac leak. A 305 engine should not have a huge pinging problem unless there another problem, even on 87 octane. When it is cooler out, everything is shrunk down. As things heat up they expand creating a larger leak. Diagnose it or have it diagnosed by somebody that can figure it out. Don't just start by throwing parts at it.