Well, according to my eyes, it's approximately* between the old and new buildings of the Boston Public Library on Boylston Street. In the attached photo, taken on Boylston Street looking (ahem) approximately east, on the right from front to back: The Lenox Hotel, The
Phillip Johnson addition (scroll down), the original
McKim, Mead, and White building, and Copley Square and Trinity Church beyond. The finish line is painted on the street around the last "column" of the Johnson building next to the McKim building. You can also zoom in on Google Maps
here and see "FINISH" painted. Copley Square may be the "official" end only in terms of that is where the runners are carried to the doctors, and where spouses go to pick up the remains. I recall the line used to be at the Prudential Tower when Prudential was the sponsor.
Regarding the history of Copley Square, it was created in 1883 and was originally a triangle lot because Huntington Avenue went straight thru it. The other triangle lot in front of Trinity Church was added a year later when they realized how silly it was having a three-sided square. The removal of Huntington Avenue thru the square was more recent.
Since the Hint was
fairly precise

and did not give a tolerance, one must conclude they meant the plaza in front of the church and not anywhere else. Unless, of course, they posted it without Dave's advice, which would be silly. I'm still leaning towards a tortoise and hare theme.
By the way, Dave, perhaps some of the official Smoot websites should be updated, as both "a salute to Smoot" at the
MIT website as well as the
Lambda Chi Alpha website indicates 364.4 Smoots "and one ear" (MIT) or "plus 1 ear" (LCA).
*Since the two engineers above are debating the issue of precision and tolerance and the possible size of Mr Smoot's ear, a lesson the rest of us should tolerate and watch from afar.
_________________
Everything I know today I've learned from listening to myself talk about things that I knew absolutely nothing about. Gracie Allen