I believe that the parking brakes on the 1940 Chevy 1/2 Ton PU has two metal pieces that fasten the cable to the backing plate. My cable has only one. It fits fine with the felt gasket and fastens to the outside of the backing plate. The cable is sloppy at that point and I believe there should be a second clamp.
We will probably need some pictures to completely sort this out.
There are 2 pieces to the clamp mechanism that holds the cable to the backing plate. Each half fits around the cable and has ears that the mounting bolts go through to attach it to the plate.
My best guess to find one is from one of who I call the real suppliers for old Chevy parts. They deal in NOS and salvage parts.
Bruce Bugay in Virginia might have one. You could try Steve at Mongo’s Garage in Missouri or Joe Mendola aka Mothertrucker in Tucson.
I did a quick check online at The Filling Station. They do have the felt seals that fit between the clamps and backing plate but I did not see the clamp.