First test, and everything worked perfectly! Well, almost everything, more on that in a sec.
I hooked up the last component to the battery pack, the DC to DC converters, and checked everything again. All checked out. I threw the main breaker and the Zilla came to life. I turned the key and all of the components came on. I slowly pushed the pedal. I could hear the motor spin, but no wheel movement. Doh! It's in neutral. Put it in gear and they started spinning away.
The speedometer works! No modifications needed. That's good news. The only bad thing is that the vacuum pump for the brakes didn't come on. I fiddled with the wires and it came on for a second. Checked everything with a meter, and it's all wires are good. It' looks like the pressure switch may be bad.
Going out to work on more stuff, so more later.