Your newer trucks must be different than my '37, I was able to drop the box and column out the bottom with the front end pretty high up in the air. It took a little wiggling, but it came out in one piece. Flushing the box might be pretty tough without removing the covers, there is only one way out besides the filler plug, and thats up the steering shaft into the cab! The '41 gear box I am using has at least one of the end cover bolt holes that goes into oil, yours might also. With that you might be able flush some of the old oil out. Fill with gasoline or kerosine and let set, working it side to side every so often, then drain out, let dry and refill.
