GM stamped the VIN on the block of a Chevy small block starting in 1962 for some engines and all engine blocks since 1968. Replacement blocks did not have the VIN for obvious reasons. Perhaps truck blocks were not stamped until later....not sure how to find out.

There is a one, two or three letter code that identifies the installed configuration, a code for the engine plant, and a date code.
On October 1, 1996, GM ceased the stamping and replaced it with a bar code label on the oil pan