You may be able to take some strain off the wiring by routing the harness on the inside of thev alve cover rather than over the headers.
I'm not sure I have any photos of '66 engine bay but someone here probably does. it will give you an idea of how it was supported originally. @toolmanmike
Discussions on cleaning here: Cleaning bulkhead connectors
I also like De-oxit and if needed on heavily oxidized terminals, I'll scrub the metal using fiberglass cleaning brushes that look like mechanical pencils.
Do a search for threads on crimpers and crimping wire terminals if you find that is needed.