The cutting part of the nibbler is smaller than the head of the tool. It will work, it will just take forever. If all you want to do is cut out the stamped grill, like I did last night, why don't you just use what we used to call "aviations". They are tin snips with kind of a compound action. They come in 3 colors, red, green, and yellow. The red are for cutting right hand holes, the green for left hand, and the yellow are straights. I don't really think it matters that much, I usually get the red ones. They cost about $15 at Home depot. I cut mine out in about 5 minutes.
]Monty[