Steve, i think you should consider loctite or something similar as a barrier so that the stainless steel bolt does not enter into electrochemical corrosion with Zn-Al spline due to moisture. If the hole was chromed inside it wasn't a problem, but after threading it's just bare Zn-Al.
Or you can just use zinc plated bolt instead.
p.s. great idea anyway
hi, not sure if electrolitic corrosion might be relevant, theoretically yes, but,...
i do not think loctite will help, when you screw the bolt, despite you use anything between thread and bolt, there is contact metal/metal in the tension areas
the best possible would be aluminium or copper grease, but yes, it is a grease
moreover it is a very small place for loctite, it might go everywhere
i would use zinc plated bolts, BTW, never saw beheaded bolts in zinc finish
additionally considering that there are three elements, (arm cap/exterior spline in pot metal/ interior spline in steel)
and moreover knowing that there is a relative position of the arm and axle; i do not see that the beheaded bolt is a good idea
easy to say from my side, being the damage as high as it is losing the wiper arms, anything is better than nothing
seriously i would prefer to put a piece of nylon string to secure the arms
good luck