73 Duster to 69 Valiant grill conversion parts needed?

Your hood will work, but you will have a gap at the top, where the relief is for the Plymouth header, on the aluminum trim that clads the plastic grille.

Technically, you're looking for the header panel that holds the grille, behind it, that has the hood latch and spring, the hood, you'll have to remove the die cast fender fillers for the '73 bumper, you'll need a '67-'72 Valiant bumper w/ brackets, headlamp bezels and maybe some wiring for the front signals. Not sure on differences in wiring length for bulb sockets. You may have to find out after the fact.