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I have been talking to visor owners who have installed them using sheet metal screws per the instructions & they claim they have never had any problems with them shaking lose. Just the thought of it, makes me nervous.
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We have big mirrors and other accessories installed on vans with only sheet metal screws. In many cases they never loosen up in 300,000 miles. When they do come loose I use weatherstrip adhesive on the threads of the screw to act as a thread lock. I've never had a problem up north but maybe it won't work in the heat in TX.
A screw is supposed to work like a spring. As you tighten the screw it stretches slightly which helps create friction at the threads. In order for this to actually work you need a strong enough piece of metal to screw into and a screw that will stretch before maximum torque is achieved. Folks often fail to realize that installing a larger diameter screw in a thin piece of sheet metal will often reduce or eliminate the chance that the screw can be stretched before the sheet metal distorts. Smaller diameter is usually better.
The 55.2-59 trucks have two pieces of sheet metal welded together at the top of the windshield. This adds strength and allows a fastener to be tightened more before the metal strips. This may be why many visors never loosen up.
If you have to use sheet metal screws during an installation, a punched hole is often much better than a drilled hole. A drilled hole leaves the metal around the hole flat or slightly raised toward the screw head. A punched hole pushes metal away from the screw head and when a screw is turned into the hole the excess metal is pulled back into the hole where it increases the amount of torque required before the hole is stripped.
Good luck and please report back on this installation, if you complete it.