1976 Dart Overheating and other issues

It sounds like you are fairly inexperienced at this.
So....DO NOT remove the radiator cap while everything is hot! I've seen 2 guys get badly scalded doing that. Imagine Old Faithful geyser right in your face. Even just warm, cover the cap with a doubled up towel before removing.

Overheating at cruising speeds sounds like a radiator problem. As mentioned already, I'd take it to a raduator shop and get it checked out.