Welcome to the forum! Nice truck, you ought to post some pictures (hint, hint!) I normally hang out on the 47-54 site, but occassionally I stick my nose in where it doesn't belong.
Your serial # breaks down as follows:
"3B" 3200 series truck 1/2 ton long bed
"59" indicates the year of the truck
"K" indicates that it was assembled at the Kansas City, Mo. plant
"125363" is the sequential number of your truck. It was the 25,362nd vehicle off of the line.
The RPO 703A indicates that the truck was originally painted "Galaway Green". FYI, RPO stands for regular production option. Each of the 14 original colors for '59 had a different RPO #.
The "123" indicates that you have a 123" wheelbase, which would make it a long box truck.
Hopes that sheds some light!
Ken