Wow, lots of questions Brian! First let me catch up with some more photos and then I'll try to address your questions as best as I have been able to find out.
First, these pictures show my old bracket, as removed, and Dean's new bracket installed. My old bracket has a clip that holds the speedo cable, Dean's doesn't, so I might still repair my old bracket and re-install it. I think this is because in some years (like '76) the speedo cable doesn't go straight to the t-case; it first goes to a secondary speedo that controls things like the EGR maintenance warning light.