Here's that part from Classic Industries Link
]. I believe other vendors carry them also. (LMC PN 38-8940 - they call it a Grill Support Panel)
They say fitment is 47-53. The FAM shows only 54 front sheet metal, which is probably why you couldn't find it.
The bottom of your grille bolts to it, and like the other guys said, you need one. You might be able to get an original from one of the members. Try posting in the Swap Meet