you don't say what model/year g.m. so.
do this. trace the old wire back to the firewall bulkhead connector most times on drivers side near brake booster/master..
I tap into that wire about 3-4" from the bulk head connector buy striping an inch of the wire (not cutting the wire) and then soldering in an other wire at the spot I stripped the insulation off the original wire.. then tape up your splice.(I use the liquid electrical tape here) and run your new wire along side the original one.. now plug your new wire into the mds hei you have full 12 volts.. AND still have the original wire incase the hei ever fails you can drop the points distributor back in and hook it up to the original wire.. and get the car home.. or if you (or next owner)want to go back to stock. just tape up the old wire's connector that hooked to the points distributor and you are good to go..
it should take you about 30 minutes to do this. after you chase the old wire back to the bulkhead connector. on firewall..

 

I've done kinda the same as killer but I've just run a new 12 gauge wire from the dizzy to an unfused spade on the fuse box. And taped the originals just in case. Carried the points dizzy in the glove box.

 

Trace the resistor wire, there may be one of two different configurations for your points which would be an external resistor mounted close by the coil or a resistive wire that runs from the "RUN" terminal of the ignition switch to the coil. There may be a second wire at the coil that comes from the "START" terminal of the ignition switch, this provides battery voltage during cranking only.

If you have an external resistor all you need to do is connect to the 12 volt side of the wire from the ignition switch to the resistor using a 12 gauge wire from there to the MSD. You just coil and wrap the old wire from the resistor to the coil in case you ever want to put the points distributor back in.

If you have the resistive wire, then you need to trace that circuit back to the ignition switch. That wire will connect to the "RUN" position of that switch. Add a number 12 gauge wire to that "RUN" terminal and run it to the MSD unit. Leave the resistive wire in place, coil and insulate cover it and securely stow it so its available if you ever reinstall the points distributor.

Bogie

 

Trace the resistor wire, there may be one of two different configurations for your points which would be an external resistor mounted close by the coil or a resistive wire that runs from the "RUN" terminal of the ignition switch to the coil. There may be a second wire at the coil that comes from the "START" terminal of the ignition switch, this provides battery voltage during cranking only.

If you have an external resistor all you need to do is connect to the 12 volt side of the wire from the ignition switch to the resistor using a 12 gauge wire from there to the MSD. You just coil and wrap the old wire from the resistor to the coil in case you ever want to put the points distributor back in.

If you have the resistive wire, then you need to trace that circuit back to the ignition switch. That wire will connect to the "RUN" position of that switch. Add a number 12 gauge wire to that "RUN" terminal and run it to the MSD unit. Leave the resistive wire in place, coil and insulate cover it and securely stow it so its available if you ever reinstall the points distributor.

Bogie

curous why all the way back to the switch, why not just to the firewall bulkhead connector, I've always done it this way, the crank power half of the circuit is a full 12 volts when cranking so the resister or resistive wire isn't on that end. at lest not on the muscle car year g.m. s I've owned.

 

I don't have a old style truck as mines a chevy s10 but I had a situation that the 12 volt source wire I used coming out of my firewall for some reason just lost power and tracing it could never find the issue. Whoever owned it before me just cut and hacked all the wires up and had not reason to do so. To make it simple and tired of having wires go bad I just ran a 12 gauge wire with a inline fuse from the battery to a toggle switch then to the hei distributor so now I have a excellent 12 volt source with no more worries. That is an option you can do as well to make it simple.

 

Got the wire bypassed and getting 12v to HEI. This was the problem and now have it running. Just having problems with what springs to use and stop bushings.
Running 283
2101 edelbrock manifold
1504 edelbrock 500 cfm
open air cleaner
HEI distributor 8365