Today I noticed that the Classic Industries catalog shows a large sheet metal panel that they show installed directly beneath the radiator support frame, for 47 thru 54 pickup trucks. I know that my 53 half ton did not have one in it when I bought it. I looked in the FAM manual and I could not find any reference to it. I looked in the Jim Carter's catalog, and found NO reference to it there either. Classic Industries calls it a "radiator support". I would call it a splash guard. I guess my question is: did this part actually exist in the original 3100 AD trucks? Their picture makes it look like it sandwiched between the crossmember and the radiator support. It appears to span from one frame rail to the other, and extends in front of and behind the radiator. Please advise.

Give us the Classic part number. 61-712 ? Yes, correct for 53. Should be called "Grille Support". 47-53........... NOT 47-54. That one is GMC ONLY. 61-712G

You need one. It forces air through the radiator. I call it a lower baffle.

Here's that part from Classic Industries Link [classicindustries.com]. 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.

Thanks for all the help. You guys are the best.