1973 duster fit 20"s??

They'll fit...and so will the following other "bolt-ons" that you might be interested in...

Curb Feelers...
Dingle-Berries on the perimeter of the head liner...
Chromed Folgers Coffee Cans used as exhaust tips...
8" Chromed Chain-Link Steering Wheel...
Fake Fur on the Package Tray (Speaker Shelf)...
NOPI decals on the windshield...
EVERYTHING in the J.C. Whitney Catalog...

And remember to bolt all 42 of the sub woofers on the rear bumper. That way they'll help to propel the car forward...

Do the humane thing; sell the car.