As I recall the instrument cluster grounds through the dash frame, if you painted the frame you lost your ground (very common issue). You can crawl under there and run a temp ground from something on the firewall to the cluster and have someone look to see if they come on. Also, there should be a ground wire (black with a ring) on the right side of the dash by the glove box (I think), this gets grounded behind the lower bolt that holds the dash frame to the side cowl. Make sure this this is clean and making contact.
As to the black wire, I would first use a test light or multimeter to ensure there isn't power to it (check with the ignition off, then on acc and then with the lights on). My 70 Challenger manual shows the black wire running to the trunk light as only one wire which means it should be hot. Even if your car didn't have a trunk light, that wire may still be present in the original harness as they may have used what they had at the time light or no light. Test it regardless, if its hot and you don't want to run the light, just tape it or zip tie it out of the way.