Good summary there 318 Stroker.
Conserning the original 73-74 front bumper, it will bolt onto the 70-72 bracket once you remove the re-inforcement plate. There will be 1 extra bolt hole on the top side of the bumper, where the big bumperette bolted on. A bumper bolt can fill that. Note: If you also change the rear to a 70-72 type, the jack slot may not match your original 74 front.

You cannot remount the 73-74 front bracket inwards on a Challenger (maybe a cuda its ok). The frame rail is in the way.