Electrical Problems (fuse box question) 1973 Duster

Not 100% sure about '73, but in '74 there are several spots in the fuse block that were not used for anything.

Headlights do not go through the fuse box at all. There is a circuit breaker built in to the switch.

It's best to have the actual factory wiring diagram when checking things like this, but this site has diagrams that may help.
http://www.mymopar.com/index.php?pid=24