The '66 C-10 wiring harness has a resister wire which feeds the coil for the points-style distributor. You do not want to use that wire for the HEI distributor. You need full battery voltage.
You may be able to purchase a wiring harness for a '66 C-10 which is constructed specifically for an HEI distributor. That would make life a lot simpler. If not, all you need to do is run your own wire from the ignition switch to the coil on the HEI and remove the resister wire all together.
