Art,
The instructions specify #22 wire. Two pieces, each 153 inches long. One is the positive power wire, the other is for the ground side. These wires crimp into the butt connectors that are just above the shelf in the instrument area. They follow the same path as all the other wires that run from the instrument compartment all the way back down the tunnel. Make the wires follow the wire that goes to the fuel tank sensor, over to the right side of the airplane at the bottom of the roll bar. Coil up the extra and leave it there for now.
I too have that coil of #18 wire left over. I keep waiting for an instruction to use it! I don't know if there is enough to do two 153" runs from that, but if there is I don't see a problem using it instead of the #22, since it is a heavier wire. Of course that would raise the issue of 100% conformance to the plans.
You could always just order some #22 wire from Spruce.
Either way, run that wire now, you will be glad you did!
John