Basically a true story, but the details are a little different. When designing the 201, installation of a six was not envisaged. The 201 design began in the late seventies and was MBZs response to the energy crisis, so only a four cylinder engine was planned, but when fuel prices dropped the engineers got creative. The 201/2.6 was first shown at the Tokyo Auto Show, circa '86. In production the early 2.6s have front end sheet metal that is 3mm longer than the fours, and you can verify this by looking at the front overhang spec and overall length. This fooled a lot of body shops!
Circa 1990/91 the longer front end became standard for all 201s.
As far as the belt change is concerned, I haven't done it, but picked up the following procedure off this forum.
"Slacken the large bolt (17mm) that goes through the centre of the tensioner. Then loosen the adjuster nut (13mm) and the pulley moves back so you can fit the new belt. Set the adjustable plastic arrow at the end of the cast triangle and tighten until it's at the other end. Finally, tighten the 17mm nut."