could a 1975 duster into a demon..

It's possible, but a lot of work.
I changed a Duster into a Dart Sport and working on a second one.

* Change the entire front clip (fenders, hood, grille, bumper, latch tray, etc) and fabricate a mounting arrangement for "regular" front bumper brackets. I believe 1970-1972 Swinger and 1971-1972 Demon bumpers and brackets are the same. 1975 and 1976 Dusters have "shock absorber" bumper brackets.

* Install a Demon taillight panel/lights in place of the Duster.

* Fabricate a new mounting arrangement for a "regular" rear bumper. I believe 1970-1973 Duster, 1973 dart Sport and 1971-1972 Demon bumpers and brackets are the same. (double check) You might be able to retain the original later Duster bumper and "shock absorber" brackets if you retain the filler between the bumper and body.

* You'll have to match up the side marker lights, depending on what year you make it. All 1972 and later A-bodies had the long, plastic, surface mounted lenses. Earlier year had smaller versions recessed in the fenders and 1/4 panels.

* A 1971 Demon may have a flat trunk lid. 1972 and later Demons, Dusters and Dart Sports had the one with a rib down the middle.

* Not sure if the rear 1/4 wheelwell lips are a "little" different or not.