timing...

It looks like a vacuum hose. Definitely plug it since it could be the source of a vacuum leak. Do you hear it hissing with the engine running? As far as where it goes, you need a vacuum wiring diagram. There was probably one under the hood when new, at least for the emission controls tubing. If gone, you can find them on-line.

By the early 1980's, engines were almost impossible, with many poorly documented vacuum hoses and devices, carburetors with kludged-on electronic controls, etc. My 1985 Mercedes is the worst I have had, with even vacuum-controlled transmission shifting. If the trunk lock gets a vacuum leak, the engine won't shut off. It makes your few vacuum lines look like a baby step. The internet is great, since people figure out the controls and post photos and schematics. You can probably find similar for your 1975 Dart.