You have to wire the new instruments to the old harness. I bought one of those color wiring diagrams and it wasn't too hard. Definitely worth the effort.
I installed one in a 69 Cuda. Worked out great. Had to trim the wiper switch quite a bit to fit. Having gauges that work and you can read is a big upgrade.
I never could figure out why people drove around with the rear windows up on their convertible with the top down. It looks funny. So its because they don't work. Stick with the manual windows, another mystery solved, thanks RedFish.
Rather than dye the old fabric another option is to have it replaced. The old fabric loses it's strength over the decades. I used Sharon's seat belts out of Texas. They can even get the metal rechromed. They look like new. It was not expensive.
Didn't the 1969 cars have them as an option? The 7/16 nuts might already be welded in under the trim. If so, you could just buy a set and bolt them in. I think the one vendor I have seen is called SnakeOyl or something like that.