Some searching online should confirm, but my memory is that 78 is the transition year. Some old (71-76) some new (79-93).

Yes, it is...
http://www.allpar.com/trucks/b-vans.html

https://en.wikipedia.org/wiki/Dodge_Ram_Van

You can check your local wrecking yard for interchange info, I know the U-Pik-It here will provide it free.