What Ed is saying is, get both back tire off the ground and fill the tranny with diesel. The side plug is your fill level.
Run the truck in second gear for a minute or two. Drain and fill with 90 weight gear oil till it starts to come out the side plug.
The fill level does seem low, but it get the lower gears in the lube and splashes the rest of the gears and bearings for lubrication.

Don


1967 GMC 9500 Fire Ladder Truck
"The Flag Pole"
In the Stovebolt Gallery
1946 2-Ton grain truck
1946 3/4-ton pickup,
1953 1-ton pickup
1957 1-ton flatbed dually,
1954 Pontiac Straight Eight
Of all the things I've lost in my life, I miss my mind the most!