[fleetsidelarry]

the purple goes to the "S" post on the solenoid NOT the big lug (battery power). the purple comes from the ignition switch (and neutral safety if so equipped) and is only energized when the key is in "start" position. it is what triggers the solenoid to activate the starter.
Since you are going to run an HEI you shouldn't need any other wires to the starter since it is no longer necessary to use the "R" post for ignition (if you power your HEI via a wire to another switch-on source as is typical).

I don't know where the green and red wires go back to under the dash. if you can trace them back, we can figure out if they are needed and where to put them. on my wiring diagrams there are no oem red and green running to the starter but I don't have a '72 diagram