Doesn't the 390 FE have enough low end torque to handle it? 4-lo and granny makes this thing REALLY steep.

 

yeah, but these trucks love the gears

 

Not to mention the 2.7x:1 low range in the modern truck's Borg Warner cases is far deeper than the 1.96:1 low range of the New Process transfer case.

A 390 FE is able to torque its way through a lot, and a 6.68:1 granny gear helps, but for the times you are not in low range or 1st gear it is nice to be properly geared for the tire size. Then again, a lack of overdrive would make 4.56s no fun on the freeway.