This particularly story was sort of a "bonus" story at the end of another Marvel comic. It may have been a Marvel Double Feature, but I don't think it was. I think it was tacked on to an Avengers comic, but that may not be the case.
The story is short, but quite clever, with Loki in his spirit form tricking Dr. Strange into stealing Thor's hammer. The deception is uncovered and the two have a very entertaining mystical battle. Despite only having a fraction of his full power in his spirit form, Loki is clearly far more powerful than Strange, and would have destroyed him except Thor manages to close in and Loki flees.
I have been able to find screen shots of the story online, but none of my Google-fu has turned up the comic it originally came from. I'm trying to find what comic it came from. It's not Strange Tales 182. I'm certain the story was attached to another comic.
Mid-to-late 1970s, unquestionably. Something ticking over in my mind suggests it may have been in an issue where Hank Pym (as Giant Man) was going through his "stuck in 10 foot form" in the primary story, but I could be way off on that.