You should be able to use the fuel pump wire for your backup lights, it will be on whenever the ignition switch is on. The third brake light wire should only be used for exactly that. Its pre-wired connected to your brake light switch and only has power when the brake switch closes (when you step on the brake pedal.

I'll let somebody else chime in on the other topics, but I think the ballast resistor should be between a wire that is hot when the ignition is on and the + on the coil. And the ammeter scheme sounds correct.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.