I was wondering if I could use the existing wire from the origional coil for the power to my HEI. what is the best way to run the wiring for HEI.

The original wiring has a resistor in it and drops the voltage to @9vdc. HEI units need a full 12vdc+ voltage to function well.
I run two additional wires from under the dash to the new HEI units. One wire is connected to the Ign. Switch side of the fuse block and to the "BAT." terminal of the HEI unit. The other wire is for the Tach and is connected to the the tach lead wire and to the "TACH" terminal of the HEI unit. (note: you can only run the one wire if you won't have a tachometer.)
The proper connectors and plugs are available at many parts stores or I get some from wrecking yards when I'm looking for other parts. If you just use spade type connectors without the proper locking plug they can fall out.