Six digit GM part numbers are common for the pre70's era. In my small block book, there are six digit numbers listed for partial engines (a partial engine has a block, crank, cam, timing chain, rods and pistons and is commonly called a short block) ( a fitted block has the block and pistons).

For instance, 376372 is a 62-67 partial engine with a hydraulic cam.

Pop the plastic and identify the engine. Grab a casting number and the engine code and we can identify it.

mike in tucson