How do I tell

what motor originally came in a 73 Duster? What VIN letter/number tells me, and what place in the VIN does that number appear? I am going to take a look at this 73 Duster that has a 440 in it, and the guy says he thinks it was originally a 360 car. I need to know how to confirm that.
Please email me asap with the info!
Thanks!
Don K