As Dave says your transmission is a 4 speed non-synchronized. It was an option in the 37-47 1/2 ton pickups and standard on the larger trucks. I have one in my 42 1/2 ton and my 37 1/2 ton. My first truck was the '42 and I wanted to switch out the trans for a synchronized 3 speed but I couldn't find one. Eventually, I got better and better at double clutching and now I don't want to change, as it is a skill that goes along with the character of these old trucks. You may not want to stick with this type but it would be the easiest and cheapest way to get your truck on the road. If you look hard enough you can find these relatively cheap as people want to swap them out. I was given one that is being used in my '37 project since the original had more water in the trans than oil and suffered the same fate as yours. You can always change it out later but the decision depends on you and what your goal is.
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix