I have 1973 Mach1 also.
This is me & my high school car.
In 1988 My first wife sold it for me with out me knowing. It was sitting in the garage for 3 years and I was not driving it. So she did not think I would mind.
12 years later I found my car again. It was abused in the the 90's. It was made into a drag car. Gutted sheet metal chop up to lighten the car up and a 460 installed in it.
This is how I found it. Bought it back for $400. I was just very happy I found it again.
Needless to say I had to find everything for it.
The wiring I bought the wiring off eBay for a 1973 with gauges, as you know you cant buy anything for a 1973 for wiring.
The factory wiring is very easy to install, it plugs into everything you need. With a little repairs & cleaned up it was ready to install.
The main thing I hate about those after market wiring kits,
THEY DO NOT COME WITH THE CONNECTORS!!!
Even for the easy years like 1965 to 1968's
They want you to use your old connectors on the set up.
I see a lot of wiring problems in my shop, The biggest one is with these aftermarket wiring kits.
People need to know NEVER USE SQUEEZE connectors to install anything.
Here is one I just got in. The wires pull apart with no effort.
If you ever insist using a after market wiring kit, be sure to solder all the connection together, and heat shrink them.