The 72-74 Challengers did not have the headlight bucket panel welded to that part of the fender and used the fiberglass grill with the buckets built in.
70-71 Challengers had a metal bucket panel welded to the fender to accommodate the installation of headlights. Some people who make this modification use a 70-71 fender (side lite makers are also different but fit the car.
Other option is to weld in a bucket into the 73 fender. Off course you would need a 70 grill, the bezels, hood latch, hood moldings, and other mounting parts that are different on a 70.
Keep in mind that many other parts are different; front bumpers and brackets- bumper protrudes further out on your car.
You can purchase reproduction buckets for a 70-71 from BEA Parts
to convert your fenders.
Someone here may be able to explain this better then me.
It is a lot of work to make this switch. This is just the work on the front end of the car.
View attachment 50433
View attachment 50432