I had one for a time. Apparently most Dart Customs were 4 door models, mine was 318/904/8.25, AC, power everything. Woodgrain dash, Custom specific seat pattern, and a fancier steering wheel.
It was a cool car, but I sold it for money to finish another project.
340s were supposedly an option, but they were badged "Custom GT". However, Custom GT badges came on bucket seat cars.