It is feasible but extremely impractical to install a 6.5' Truck Bed for a 2011 GMC 3500HD. The GMC Sierra 3500HD of 2011 has a frame that is made specifically to fit the 8-foot bed, and thus, it would take a lot of modifications. These involve changing the chassis, changing mounting points as well as re-wiring the components. These transformations may jeopardize the safety, structural integrity, and resale value. The modification is not common and is discouraged by most owners because the cost and complexity of this modification are more than the benefits.