picked up 1972 dart 340...............real?

Just picked up a 72 dart laying in the field. Come to find out the vin tag reads.. Lm29h2b######. It says b in 7th digit meaning hamtrack as where it was built. But reading they say they were only built in Canada??? No fender tag or build sheet. How to prove it to be real. I have title and vin tag is still on the car. Any help or I to would help. Thanks