The MC-6 subfleet was used on US101 in California and was used in Western Canada. Were there other places where they were used? They weren't street-legal on the Interstates.
The two prototype MC-6X pair, ran with wide load permits on the New York City - Chicago nonstop express, via the Chicago Skyway, the Indiana Toll Road, the Ohio, Pennsylvania, and New Jersey Turnpikes, and the Lincoln Tunnel during Autumn of 1967.
When the 100 production version's debuted in 1969, they were used where allowed...on the New York City -Philadelphia nonstops, on the New York City to Buffalo nonstops a couple of those trips continued on to Toronto, Sudbury, and Calgary), operated by pool partner Gray Coach Lines, Buffalo to Sudbury, thence Greyhound Lines of Canada. Gray Coach was the only non Greyhound company to operate those MC-6's in their initial life, although Greyhound owned all of them. Greyhound of Canada owned 15 in total. Some were used on trips as far as Vancouver, BC. In addition, some were used in Florida, some were used on the lowly San Diego - San Ysidro local, even having a cash fare box installed. One, was used on a Fargo-Pembina-Winnipeg local run, linking the two cities of its birth. I believe they also used a couple on the Baltimore - Washington local run, via US-1.
At the same period, 1969, Continental Trailways acquired 45 Eagle Model -07's, which also were 102" wide. Most of those held down the New York City - Philadelphia run, bus several others ran between Pueblo, Colorado Springs, Denver, Cheyenne, and then either Rapid City or Billings. I believe they also had one or two on their Baltimore -Washington local.
These coaches paved the way for the eventual legalization of 102" wide buses, nationwide, and that is the standard today.