4 wires left!

[1963chevyc20]

okay. so about a year ago my truck almost burned down. wires were smoking every where. i have replaced all of the burnt wires. but now i have loss of spark. now i know this is got to be my starter harness. threw the dash there are 4 factory wires. purple black red and green. these go to the plug at the fire wall. now from there you will have wires black taped together all the way to the starter. well i did away from the wires that plug into the firewall and run to the starter. how do i re wire it back. all i need to know is what color to what post. there are 3 posts on the solenoid. S, B, and R. which wires go to which post? also i would like to know how to install an after market tach.