The odds are good your truck may already have the heater installed, you just need to buy the cord.
Ford only part as the heater is an application specific connector.
From the Ford Super Duty coffee table book:
Engine Block Heater:
The engine block heater is located on the right side of the engine block. The block heater uses 110V AC to heat the engine coolant in cold weather climates. Use the engine block heater must be used whenever ambient temperatures are at or below -23°C (-9°F). The engine block heater is standard on every engine.
The power cord is an optional accessory.
These are the Sates that trucks ordered for came with the block heater as standard equipment.
AK, CO, IA, ID, ME, MI, MN, MT, ND, NH, NY, SD, VT, WI, WY
There are aftermarket kits availbel, the most common being a magnetic pad that you attach to the oil pan.
But, is your 6.7 an early version with the plastic pan???