I am trying to determine the date/type of a transmission in my 70 challenger. Where are the codes/stamp located on it?

Thank You

 

What motor ?
BigBlock 727 only possibility
340 usually a 727
318 could be a 727 or more likely a 904

 

The codes should be stamped on the driver side of the transmission (assuming it's installed) just above where the transmission pan mounts to the transmission. So, to further explain, the numbers should be just below where the linkage connects to the transmission. It's a lot easier to clean the area off to see the numbers if the transmission is out of the car. Normally there's fluid, dirt, grime in the area and a thorough cleaning should unveil the numbers.