According to the wiring schematic at the following link, it appears the yellow wire to the starter solenoid goes to the positive side of the coil.
http://webhosting.web.com/imagelib/s...pf4&title=1966 8cyl. Engine Compartment Wiring Schematic
If the ground wire on the firewall is braided or heavy like a battery cable, it probably bolts to the back of the cylinder head. This is designed to help with grounding the body and dash instruments.