danorris,

What year/model truck?
Do you have the original rear end?

Additional information:

Patrick's or Mothertrucker's 3.55 Ring & Pinion gears in a 1/2 ton enclosed drive-shaft (torque tube) rear end (if 1937-54)

The Truckstell (Tip-Toe-Matic) overdrive unit (0.73 ratio) requires cutting the torque-tube and enclosed drive-shaft.