Someone may correct me if I'm wrong as I haven't got to this stage yet, but I believe the foam goes between the inner and outer fender near the front just to the inside of the headlight bucket. It fills the gap between the two fenders near the top of the front fender brace. I know when disassembling these trucks that foam turns into black tar like goo that is hard to get off.

1970 Chevrolet C10
Grandpa's -- My first truck -- In progress to shiny
Follow the build in the Project Journal
1950 Chevrolet 1-Ton Dually
"Ole Red Girl"
In the Stovebolt Gallery
More pictures here
1951 GMC 9430 1 ton dually--Shiny! | 1972 Chevrolet C20- Rusty- the puzzle box lid for the C10 | 1962 AMC Rambler American- my wife's
Parts trucks-
1951 GMC 9300 | 1951-GMC 9430 | 1951- Chevrolet 1300