Never installed a Fulton Visor but:
1. If the #8's look beefy enough to hold the weight, then use #8.
2. The length needs to be twice the total thickness of the parts. Too long, and it will interfere or punch thru somewhere.
3. Each size, like # 8, etc, only has one pitch in sheet metal screws. #8 is about 15 TPI.
4. I assume the Fulton has mounting holes and the truck does not. The size and type of holes in the Fulton tell you what size and type of screw to use. Use a size that fits in the hole with a little slop. Use a flat head screw if the Fulton hole is countersunk. Use a pan head of not. The screw you show may not be self tapping and will require a pilot hole in the truck, about 1/8 (check that number before drilling).
5. You may want to set up a test set. Get some pieces of metal with the same thicknesses as the subject. See what pilot works and what screw works to your liking.
6. Do not use power drivers when final tightening unless you know what you are doing.
