46 In Progress beat me to it (while I was composing this), but I'll elaborate.
1.) 3837004 is your block casting number. Most casting number lists show this as a 55-57 235 block. OM is probably really GM. IC is unknown. Usually one would find a casting date code, which is a letter followed by 2 or 3 numbers.
2.) B46 may be the head casting date code. B46, if a date code, translates in this case to February 4, 1956. There is some question whether or not this number is really a date code, though. The significance of the other numbers, 848 and 850, is unknown, though this is an 848 head (see #6).
3.) See #1.
4.) 6KD09 1002 is the truck's serial number, and it decodes as follows:
6=Oakland, CA assembly plant;
KD=series code=1940 3/4 ton truck;
09=month of assembly=September (1939);
1002=truck build sequence number. Numbering started at 1001 at each plant for each series, so this was the 2nd KD series truck off the line at Oakland.
The 1940 trucks were introduced in October, 1939.
5.) 0300933 F56Z is the engine serial number:
0300933=engine build sequence number (starting at 0001001 for each model year at each engine plant);
F=Flint plant;
56=model year;
Z=engine ID code=regular production 235 car engine
6.) 3836848 is the cylinder head casting number, known as the "848" head. This head became the standard 235 head from ~56 on.
So, your truck is a 1940, but the original 216 engine has been replaced with a 1956 vintage car 235.
