71 C-10 motor questions

[Hennyhog]

Howdy,
Bought a 71 C-10 a few months ago and have been going through it changing out parts and addressing each and every issue as they come up. I have been trying to decipher through many of online sites the numbers off the block and haven't gotten to far. First off the VIN stamped on the block hasn't any letters (151175489) in it which is stumping me. Secondly I can figure out the first half or so of the Engine code (K 03 13 CMJ) K= ST. Catherines, OT and made on march 13th....have yet to figure out the CMJ part. These numbers were taken off the front of the motor, passenger side, below the head. This isn't the original motor and I'm fine with that....just want to know exactly what I'm working with/on. Any help or info from ya'll would be greatly appreciated!

Thanks,
Henny