I used classicparts.com They have the filler panels and the seals. FRONT FENDER TO CAB MOUNTING PANEL and FRONT FENDER TO CAB SEALS. Maybe not purest, but I used rivets to attach the panels to fender prior to final install.
~ HB 1966 Chevrolet K-10 | Ghost: formerly Flappy Fenders | In the Stovebolt Gallery 1962 Chevrolet C10 1962 Suburban