Assembled or not, you will need a helper to hold it while you carefully hammer and dolly the dents. Although the process is similar to the ideas in the
PICK AND FILE thread you have the luxury of working with slightly thicker metal that you can use filler on.
Use an anvil or clamp the dolly in a vice. You will need one hand for the hammer and the other for the "corking tool" and two hands to hold the grille so with two people that leaves you one hand short.
It's going to be slow going but you will get a LOT of satisfaction from restoring rather than replacing.