This might be a place to check out.Ignition problems
Since you have had electrical gremlins, i.e. Alt, regulator, ammeter, ign switch and a replaced starter. I would:
1. Walk away from it for a bit to get calmed down.
2. Research the way the wiring is supposed to be.
3. Go back and methodically confirm wiring for each component throughly one at a time. Shotgunning for a solution just makes headaches.
4. If you are running an MSD 6AL or like system, it will not run at 6 volts. IF your 6 volt reading is during cranking I would back trace that wire to its source. resister or resister wire come to mind..
5. Where did you place your jumper? Both sides. If you jumper supplied a full 12 volts where it was supposed to be on a 6AL you should have run...unless you had a bad module in the distributer....I just replaced one, same symptoms. What ignition system you running? I don't remember reading that.
5. Be methodical, it helps.