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The old warning light style engine harness
should have had two connections to the starter: (1) The purple start wire to the "S" terminal and (2) the ignition resistance wire & yellow wire to the coil that join up at the "R" terminal. So it sounds like someone might have already done some re-wiring of the ignition circuit. But that shouldn't be an issue since those wires get removed when you modify the harness for HEI anyway.
There are quite a few places to buy automotive wire and the Packard/Delphi 56 series terminals and connectors. Some sources include:
www.terminalsandconnectors.com,
www.waytekwire.com,
www.wirebarn.com, and
www.clipsandfasteners.com Electronics parts suppliers such as
www.digikey.com and
www.mouser.com carry the terminals as well. For in the engine compartment, I would recommend using wire with a high temp rated crosslinked polyurethane insulation such as type SXL, GXL, or TXL.