The SM318 3 speed has never been known as a "granny gear" transmission. It does have a fairly low first gear which is not synchronized.
However, the later 3 speed full synchro Saganaw transmission and some of the car 4 speeds have similar first gear ratios.
Many of the engines of that era were 6 cylinders which required lower gears to make power at low speeds.
The factory 3 speed with a BW overdrive (SM319) is a bolt in for those trucks.
Fred
