Classic Auto Air kit in a 74 Duster.

I have Classic Auto in my 71. Purchased mine over 2 years ago. It needed a bit of changing to fit cause they used a 68 barracuda for a template and the radiator saddle area was a bit different. I know there were a few changes after I bought mine to make it a little more universal. (Some hoses instead of hardlines).
I suggest call and ask them why it won't fit and see if you have the ability to make any needed changes.