Maryland muscle,
The VIN derivitive was stamped in two places on top of the left hand frame rail. One is about 6" in front of the transmission crossmember, roughly under the front edge of the front seat. The other is midway along the straight rail that runs along side the trunk well. There's not much room between the body and the frame, but you can get a small wire brush, or your finger and a rag with solvent in there to clean it. You'll have to use a flash light and a mirror to see it.
By "derivitive", I mean it will have the Letter corresponding to the assembly plant (such as "B" for Baltimore), followed by the last 6 digits of the VIN. No additional or hidden VINs on the "body" nor on the blocks in '61.
You did not say whether this "bubbletop" was a Belair or an Impala. And as Ollie said, if it's being represented as an SS, you need to do a lot more homework than checking the VIN. Perhaps posting all the information from the cowl tag would be a good idea.
Verne.