Noted automotive historian Richard Langworth wrote an extensive piece on McCahill in Automobile Quarterly a number of years back, then revised the article for Collectible Automobile a few years ago. Collectible Automobile sells copies of back issues, and this one could still be available. McCahill apparently had no children and was supposedly the last surviving descendant of the historical Scottish figure Rob Roy.
Mechanix Illustrated (MI), published by Fawcett, never really seemed to recover from the blow of McCahill's death in 1975, and as the years went by it covered the car scene less and less. It was always third to the more established Popular Science and Popular Mechanics anyway, and a sister Fawcett magazine, Science & Mechanics, did even worse and folded as a monthly in 1975. (Special issues of S&M appeared occasionally into the 1980s. In its heyday in the 1960s S&M was almost like a sensationalistic version of MI.)
Fawcett sold MI to CBS, and the name of the magazine changed to Home Mechanix in 1984. Little about cars appeared after the name change. Later the magazine became Today's Homeowner and finally folded for good in the mid-1990s. The publishers of the magazine This Old House, which is linked to the old PBS TV series of the same name, evidently now have the MI rights and, presumably, the assets, according to what I found on the Internet a few years ago.
Thought you might find this interesting. MI's fortunes were tied to McCahill's. When he went, it was just a matter of time before the magazine—in the form it had then—followed.