MSD, TCS, and Other Four Letter Words

If the factory power wire for the coil has had the resistance wire cut off you could get another one off of a donor car and maybe get it completely all the way to the connector in the bulkhead fitting. You would then have to unwrap the engine harness loom that's in the car now to remove the old wire and put in the new one but at least you are not replacing the whole loom unless it's buthchered up real bad and needs more than just that one wire replaced.
I put in a factory tach in my 74 Nova that didn't have the wire run from the coil negative terminal to the dash but I was able to use some wire and terminals off of a donor harness and it's undetectable and I didn't have to put in a whole new engine harness as the rest of it was just fine. I just had to spend the time to do it. I even made sure I used the right fork terminal for the coil wire ends.
I would evaluate what you have now and if it can be fixed easily, then fix it. If it's too far gone with a bunch of splices here and there, corroded terminals and such, replace it with a whole new assembly.
If you are getting your ducks in a row, get a donor harness or a new one and just swap out the whole works when you get to that point.
There's nothing worse than trying to fix a harness in the car with soldering and wrapping it and if it's on a bench for repairs it will make it so much easier.
Jim

These may be some things you already know but to make swapping the distibutors easier, mark where the plug wires and rotor are at so when the new one (old style) is put back in you will be close on your timing.

The old style distributor will only have one wire coming out of it and this will go to your coil's negative terminal. If you are running a tach then there should be a wire off of the MSD box that goes to your tach. Connect that wire also to the coils negative terminal.
Here's where you may have a wire wire to run if you want it back the way the factory did it. Off of the starter there is an "I" terminal. A wire needs to be run from that stud terminal directly to the coils positive terminal. This wire will only be live while cranking the engine and give the coil a full 12V for easier starting. Off of the positive coil terminal a short wire needs to run to one terminal of the clay resistor. The opposite side terminal on the clay resistor then goes to a true ignition source which may be already there with the MSD box. This wire needs to show voltage while cranking AND when the key is left to return to the RUN position and then goes dead when the key is turned off.
How this all works: When the car is cranked the coil sees a full 12V off of the "I" terminal on the starter and when the engine fires you let off of the key switch and let it return to the run position and now power is going through the clay resistor to the coil which keeps from burning up the points and allows the engine to keep running.

Jim