Yeah drop the tank and the filler neck should come out as you pull the tank away. You might want to refasten the neck to the body to hold it in place.
If converting to 73-76 Dart MC and manual brakes, but run it manually and you will need a manual brake pushrod. Mopar Perf has an adjustable one. Some people complained about that part not fitting right butI have no personal experience with it. Maybe someone here has a stock manual brake push rod available?
But now I see you bought an RMS manual brake kit. If it come with a push rod, you're good to go, or stop.