Definitely custom, and not very good stye wise in my opinion. That rear glass makes it look like a hearse. In 63 Chevy made the Impala, Biscayne, and Bel Air wagons. All were five door using the same body. The Nomad was the three door produced as an A body from 1955 to 1957, along with the Pontiac Safari. Then they went to a five door B body.
I may have put the dashboard in that Mustang. Worked in the Ford plant there from 64 through 67 spot welding dashboards in like a trained monkey before starting the trade I followed for almost 50 years.
Ruined that puppy. I hate those lowered cars that scrape the road unless its a sports car designed for handling.