First off, pull the Negative off before ever messing with a motor.
If your repair manual did not say that, you need a new manual, a real one.
I had an Original 1967 Mustang
, I bought it from the original purchaser with papers in 1978, that car still runs like a champion, original 300ci 6cyl Canadian Forged Ford Motor. 1 bbl Carb, 3 on the floor. she can out torque almost any car that comes next to her in the 1/8th, sheI own them, that is all original, I rebuilt the motor at 240,000 miles, used OEM Ford parts.
You have just about managed to ruin a very very sweet ride.
But that is your thing, so go for it, I suspect a Fuse Block, or Fusible Link , which were not made in 1967, but sounds like one of them add on electric components and all that goofy wiring that was not needed is what pooched the electrical system.
You need to have the Negative Terminal off the battery.
Positive Lead off the starter.
That is like car repair 101 stuff, if there is no ground, there can be no ignition.
If there is Ground, a circuit can be completed by any slipped wire or dropped metal object that touches Ground.
I have seen people do that , I have seen them almost lose digits when the car all of a sudden cranked.
Get a Haynes or Chiltons Manual, get both, read them. They will say Remove the battery. That knocks out any chance of it exploding in your face or starting the crank on a whoops.
Now that you have read that manual, read the Manual that came with all those junk add ons you did not need.
After you have removed about 25 lbs of junk, you might be near an original Mustang.
Wait a sec, you never mentioned the engine, ut oh, you went a bought a chop shop Mustang.
Ok, so work with what you have, you have a 67 Mustang Body with an unknown ford motor.
I wired my 67 twice, the two times it needed it, I never added anything to it, I never took anything off that was still working.
The column had three wires, Hot, Ground and Neutral.Or Positive, Negative and Neutral.
The only way to read the Voltage/Wattage and Amperage on a battery is when the battery is under load.
Go up to any 12v battery, and dry test it, yep, it has power!
they all do.
Under Load , when the motor is running, is when you test it, that way you will see if there is a bad draw, too much in, too much out, you know the stuff that is not right.
Then you can determine where to look at next, if the batt is overcharging, alternator is over amping, if battery is losing charge, is a short, a bad connection or alternator is worn out.
Those old cars are so simple, do not over-complicate a simple 12v system.
That is just not right and ruins any collectible value that car may have once had.
Sorry to be so but c?on man, is a simple 12v (unknown CI) Ford Motor.
Thats 50 years of Ford Mustang your looking at, Lee Iacoca was a genius!