The '70-'71 Darts use a BA9s (SC mini bayonet) base bulbs, originally type 1895, for the side markers. The socket + wire pigtail are integral to the housing.
If your front housing has an empty hole in it, either it's a rear housing or a hacked front one. You could try using the rear-type socket/boot 2983 545, tho it
looks like it's for a bigger-than-5/8" hole.
While you're at it, move the (front only) side marker lights' ground wires as described
here to add side turn signal repeater function to them.