Steve,
I've found all of mine under the bottom section of the back seat, as well as hanging from the springs in either of the front bucket seats, I recently found one under the back plastic "shell" or cover on one of the buckets...but low and behold it was for a 70 SS which had a production number @10 cars ahead of mine. I've only ever seen pieces of them under the gas tank.
The reason I suggested checking the casting date on the block, was for the posibility of someone re-stamping an incorrect block.
I don't think you'll get any confirmation from GM, unless your car was built in Canada..in which case they can provide documentation on it for a fee.
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Dave C.
70 402 4spd Monte Carlo
71 SS 454 Monte Carlo
72 350 Monte Carlo
69 427 Corvette roadster
86 Grand National
96 Impala SS