Generally, the garnish molding screws are # 8's or # 10's. As far as length they go all over the place from about 1 inch to 2 - 2 1/2 inches depending on location.
Most garnish moldings use an oval head screw. Check the indent of the screw hole for size (should fit down into the indent)
Usually head size on trim screws are the same on all screws of the same diameter ( # 8, etc). However, there are trim screws that have a smaller head size than standard, such as a # 10 screw with a # 8 head.
Not familar with the sizes of the trim screws on 56 Ford.