Meanwhile... I've been trying to figure out how to do the ground stud through the firewall (overthinking it basically... ;-)).  I was hoping that some AN6 bolt would work (because of the Cad plating), but there's none that have enough threads to do the job.

Trying to find a cadmium plated threaded rod didn't yield much... Basically because cadmium plating is limited to a few industries like aviation (corrosion resistance).  I'm sure you know why cadmium plating has become rare - cadmium is highly toxic...  I think handling our AN bolts is not much of a big deal because most of the toxicity comes from inhaling it or, and I certainly know this, using cadmium paints and having that absorb into your skin.

I know about cadmium in paint because when I'm not building airplanes my other hobby is painting.  I've always been super cautious when using cadmium red for instance.

In case you're curious, this is one of my paintings.  It's a barn in Concord, MA (not too far from my house).

Anyway, I'm ordering some stainless steel threaded rod from here (1 ft. of high strength steel 3/8"-24) as SS is perfectly fine to use and doesn't have any issue with galvanic corrosion with aluminum.