I just got a 65 impala w/ 283. I was getting no spark. I replaced coil with accel ignition coil still no spark. I checked positive side of coil with multimeter getting 12v in on position also showing 12v on neg side of coil as well. I know it should only show 8v with switch in on postion. I held ignition wire to firewall and no spark. Do not know what to do next.
Holding the wire to fire wall (which is painted with a plastic paint and as such insulates it electrically) is not a good idea. You will blow out the resister wire that is supposed to drop the 12 Volts from the ignition key to only 8 volts to extend the life of your points by doing that.
12 volts at the coil is Abbey-normal if the factory resistor wire is still in use. But not if some one switched over to an HEI distributor in the past, and replaced the resister with a copper one (the resister is a nickel-chrome wire).
There is also a wire that feeds up from the starter relay that supplies a full 12 volts to the coil during cranking, but only while the ignition key is in the start position. In your case a previous owner may have held the wire to the fire wall after he blew out the resister wire requiring him to find a substitute, such as a regular copper wire. Even though most auto parts stores can sell you Ni-Chrome wire in a fourteen inch length to replace a blown piece. You can also buy them as a service part from American Auto Wire on line.
You would expect the voltage to be on the ground side of the coil as that just tells you that you have continuity in the primary coil windings when the points are closed. Now if your points are really badly burnt you will always have closed points as they can never open with the amount of material transferred from one side of the face of the point to the other.
I would replace the points and the condenser to see if that cures the issue of constant voltage. Also check the condition of the cap looking for cracks or carbon tracking between the terminals, and the rotor for erosion.