How to verify if an A-body is numbers matching?

RUST is what you need to really watch for in body condition. The transmission is also stamped. Location is on the bell housing where ni mates up to the motor. I am thinking on the drivers side.

Here is a very detailed article about what to look for. It is © so I can't repost any of it.
http://www.mmcdetroit.com/Buying_A_Car/