So...I am working on a rat rod for a body shop, VIN is an 84 firebird,  body has been cut off and an old chevy truck dropped over it.  It has a small block chevy in it, 400 crank, 350 block .040 over, I got to checking engine ID numbers and stuff wanting to figure out what it came out of..  so far this is what I have;

Casting number 3970010 block number on left rear of block, nothing special, GM made a gazillion of them, may have 4 bolt mains, (probably does, haven't verified yet, waiting on my borescope to show up for a month now  : )

Engine ID stamped on right front of deck is V0912CJM, figured out V=Flint MI plant, 09=Sept, 12=12th day of Sept, CMJ has 1 option in this case, 1974 police car  (Yes there other CMJ options but not for the year)

Partial VIN stamped on it is 15J118534, 118534 is a sequential number from rolling off the assembly line and apparently there is no way to determine what the car is by this, J=Janesville WI plant, 5= year of vehicle 1975, 1=engine, more specifically what HP it had and displacement, but nowhere can I find anything about a 1 engine code, all GM 1975 engine codes are letters, this is what is driving me crazy.

Did GM build engines at the Flint plant and ship them to other plants for installation into what ever vehicles? Would a Police car engine have it's own designation outside of all the other engine designations?  I'm guessing the date difference would account for the engine being built in Sept 74 then installed in early 75?  The Janesville plant made fullsize cars that would be used as police cars, my best guess is that it was in an Impala.

None of this really matters, I just think it would be cool to know what it came out of.  Maybe the block got decked sometime and somebody stamped bogus numbers on it to   with me    Anybody got any other ideas?