Here are a few thoughts/facts ideas/speculation/guesses
Most 47-55 trucks had the tail pipe end at the rear of the truck.
The tail pipe bent downwards approximately under the rear bumper
That resulted in fumes and noise being directed out of sight/sound/smell of the driver.
Presumably, most people turned their trucks off when loading unloading the bed/back?
Nonetheless, with a tailgate down, the end of the tailpipe was a few feet away from a person standing at thear.
Panel body trucks had the tail pipe end on the driver's side, behind the rear wheel.
The tail pipe bent downwards.
That resulted in fumes being directed away the rear doors area.
This kept noise and fumes away from the rear while driving with a suburban end-gate open while driving or standing.
This also allowed panel "delivery" trucks to stay running while opening/shutting rear doors for stop-and-go deliveries.
Rear door "stops" on panels allowed backing up right againt a loading/unloading dock.
These are just guesses - I cannot recall seeing documentation.
I'll check a shop manual.
