As simple as our cars are compared to a 2016 Mustang, there is still a lot of complexity in the wire harness's. "Murphy's Law" states that anything that can go wrong will, and usually does. With the newest of our cars now 43 years old, sending your wire harness to midlife is better than insurance. I've been to salvage yards "Parts Gathering" and have seen some unbelievable electrical repairs. House type wire nuts, yellow and crumbling paper masking tape on splices, melted wires where there was nothing on the spice, and some big time butchering where all those K-Mart blue light special radio's were installed! An under dash fire is a lot harder to extinguish and if at home in your garage, then you've got an even larger safety issue.
One of my non tach Mach 1's is off the road now waiting for my money tree to mature. When I go back with the tachometer cluster, the wire harness will definitely be going to Midlife. The peace of mind you'll have knowing your "Money Pit" is not going to burn to the ground is worth the money spent!!