Be careful what you put in or around the carburetor that there is no possibility of it being sucked into the engine.
Your clamp looks like it has bottomed-out (see picture with red circle).
GMC parts WIKI, for 1950, shows "clamp - air cleaner tube" part number 2246836 (see picture with red triangle).
I could not find this GMC part.
Jeep has a Carburetor Air Horn Rubber Seal (see picture). Maybe you can try this. May need to trim the height.
Jeep parts website and here at
https://www.ebay.com/itm/400712210310.The Zenith air horn diameter for Jeep and your GMC should be the same.
You can also post your question in a GMC truck forum.
