I'm camped in a place with no cell signal, so only getting online occasionally until I move someplace else.
I enjoyed reading your thought process and solution for mounting the switch. Very nicely done!
I imagine your Ford start battery is different from mine (cause mine is 97-newer which has a different engine and the start battery on the passenger side), but the way Ford did the battery cables on mine was .... well, they weren't just cables, but instead, "harnesses." i.e. more complicated and more expensive
On the negative side, there are/were two cables that terminated in a "double schnozzle" at the battery post. One of the cables (long one) went down under the rig to the starter (which is under the cab a ways), and the other went about a foot into the front corner near the headlight and was the chassis ground.
The positive setup was similar, with two cables merging into one at the post.
I ended up cutting both of those off and putting regular ring terminals on, then using the post-to-marine-terminal posts. I also upsized the negative to chassis ground since it was now part of my combiner loop (had to bed a 10mm wrench to get in there - they must have done that prior to building the front end
). I also found a break in the insulation and a very green section of wire down by the starter where they had "bent" the wire. That's when I found out about "harnesses" as opposed to cables because my first thought was to replace the cable. I looked up the replacement "cable" to see what it was like and found a huge, multi-headed harness instead 
Not only complicated and expensive, but it would have had the same geometry in the "bad spot" that led to the problem. So instead I cut off the bad section and used a heavy crimper and FTZ butt to add on a new section including the ring terminal to the starter. I eliminated the sharp bend that had caused the problem. Much faster starting after that!I did a similar thing on the positive side because Chinook had two MORE wires added to the Ford double schnozzle, and it wasn't pretty. So now I have two Ford rings on the marine post, plus the combiner wire on the MRBF. The fourth wire "went away" as it was the generator start which I moved back to the house bank.
Okay, that got a little off track. Back on, I have really enjoyed reading your write-up. You didn't let any confusion stop you and it looks like you are doing a very tidy job with quality components. NIce!
Oh and yes, while Blue Sea makes many of their own components, in other cases they use industry standard parts (Contura, the fuses that go in many of their components, etc.). That can be good to know, as you found.
BG
PS: I've never had any photos come up sideways, so I'm not sure why that happens.