I just skimmed through that article, and what they fail to tell you in it after enlarging the holes in the gage housing is to enlarge them both enough or at the very least enlarge just the plus side stud hole enough to isolate the plus stud coming out of the volt gage so the plus side stud doesnt become a ground short to the gage housing and melt all your wiring.
I did this mod on my 67 barracuda used a sunpro volt gage. The volt gage comes with plastic grommets for the mounting bracket that comes with the gage. What I did was use one of the plastic grommets on the plus side hole of the gage housing I enlarged. I used the plastic grommet along with plastic washers to isolate this side coming through the housing.
The volt gage negative post I grounded directly to the cluster housing using steel washers, and the lock washer and nut that came with the gage since the cluster housing is grounded into the dash frame anyways. This takes care of running an extra negative wire.
The positive wire, I ran a short length of wire to the ignition switched 12V feed going into the IVR (instrument voltage regulator) on the circuit board. On the rallye dash it's a stud going into the fuel gage. On a non rallye dash you can probably solder it right to the 12v feed on the plug in IVR on the back of the circuit board.
Pic shows a red positive and black negative wire. I eliminated the negative wire since the gage housing is the ground, and the negative stud is grounded to it. I only left the red wire. Smaller wires in pic are for a solid state IVR I made to bypass the broken internal one thats inside the rallye dash gas gage. Pic is of back of rallye dash cluster.
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