The front bumpers from 74 - 76 had different mounting holes from the 73 bumpers which didn't use shocks. The rear also had different holes as well. The front filler panel will be needed which is very different from yours, same goes for the rear.

 

Go to this site
Need 1973 Duster Front Bumper Measurement (at http://www.forabodiesonly.com/mopar/threads/need-1973-duster-front-bumper-measurement.355003/#post-1971324537 )

 

The front bumper for the Dart will be different even for your model year. The Dart grilles had a protruding center, akin to some Pontiacs and '70 model year Fords, which extended to its bumper. The Dart rear bumpers from '74 to '76 should match.

 

Try re-chromed bumpers. many also straighten them back to original. perhaps your bumper can be fixed.