the seals [actually 'anti-squeaks'] go underneath, won't fill any gaps ....
see here filler would be a very last resort [like someone said about adding a welding rod along the fender edge] .... I'd loosen all the front parts and see what can shift around, even a tiny bit of movement on the core support mounts might make the difference, and up/down there or the cab mounts is possible too .... the way these trucks were built, the entire doghouse was assembled to the cab and then that combined unit was put on a chassis
Bill [the untrustworthy one
]
