Knowing that my own coordinates are in the same ballpark, it seemed plausible that Wilder placed the fictional town somewhere in New Hampshire. In reality, he did something a bit more clever: he placed it just about as close to New Hampshire as he could, without being on land. It may never be known whether he realized that in doing so, he placed it at the very tiny "Dodge Rock" in Rockport, Massachusetts.
The play gives the coordinates of Our Town as 42.40°N 70.37°W. Dodge Rock is the nearest feature, with an elevation of 0 meters (watch that boat hull!) and coordinates, according to the USGS, of 42.6668°N 70.6162°W. Because it is submerged, the rock does not actually show up on the satellite image below or on most maps.
View Larger Map