I used staples that came with my cats whiskers from The Truck Shop. Packet of staples are also available from American Classic Truck Parts. They hold the cats whiskers tight to the window garnish and are invisible.
Here's how I did it. Don't know if it will work on '47-'50. My truck is a '54. 1951 through '55 first (vent windows) had fatter interior window garnish than pre '51 truck without vent windows. The thinner garnish my not have enough space to use the 4" mini-vice grips.
< CLICK > Here is a '47-'50 non-vent window interior garnish on the LEFT; and a '51-'55 1st vent window garnish on the RIGHT.
If you use pop rivets, you may want to dab some flat black paint on them to blend with the fuzzy. They will be visible through the glass from the outside of the truck.