I'm going to be doing the same thing..

I was going to custom fab some brackets out of the pile of OE brackets stacked out back, remove the iron from the rear of the bumper, weld in the studs & grind down smooth the face of them, section the bumper to bring it into the side of the car, then have it re-chromed.

I'm planning on doing this for the front & rear...hopefully a nice subtle modification that will appear stock when complete is what I'm shooting for.

The plastic filler panels on those cars are all warped and look like hell.

The cheap option would be to buy 70/71 bumpers & brackets and bolt them on and remove the filler panels, but it'll be obvious and the front pumper won't match the curves or the edges of the grille.

if you have a 74/75 Dart Sport, that was the ugliest front grille ever made and I don't think that 70/71 Dart bumpers would even remotely look good..

Another option would be to find a feather duster and rob it's hood, decklid, and bumpers.