Bore rust

First off, WD40 is not a lubricant. Never has been. Never will be. It is a moisture remover and repellent. That's why it was invented. Lots of people use it wrongly. WD40 will completely evaporate fairly quickly. So you want to use something such as PBlaster, Sea Foam Deep Creep, or even Fluid Film.

Here is Fluid Film.

https://www.oreillyauto.com/detail/...1968/plymouth/valiant?pos=0&manufacturer=true

This is exactly what Fluid Film is made for. That said, that's some very mild rust. I would soak that down with one of the above mentioned products and hit it with a ball hone and go.