Late 40s and early 50s?
Adjusted for inflation, gas was about 30% cheaper back then compared to now.
Also, farmers most likely did not drive trucks very far or for very long times.
They needed low-speed rear ends (and, 4-speeds with sub-low first gears) and relatively high torque (for carrying/pulling heavy loads and for pulling stumps/dead-animals/etc). The stovebolt engines and 4.11 rear ends were just what the farmer needed to order.
At the speeds the trucks were intended to run (and at the speeds that farmers ran them), there was no need to consider streamlining (although, the Advance-Design body was streamlined in comparison to prior bodies).
