['68OrangeSunshine]

I don't think there's a heavy Red wire in the stock wiring bundle that comes over the hump from the bulkhead connectors. One Red line comes out of the other half going to the radiator wall, and feeds the EVR and Alternator, splits and another part goes all the way along the front gutter to the pass side fender terminal and joins the fusable link off the batt pos lead.

Could it be a 12 vdc feed for HEI that a PO laced into the harness? Especially large gauge. Hotrodders used to say '12 ga wasn't big enough, ya gotta use 10 ga.'

If you have the 1968 Chevy Truck Service Manual, the L6 schematics are on the next page from the V8 plans.
A factory line to the Temp Sender should be Green. Don't know about electric choke on a Monojet. Mine was mechanical/manual.
Year of truck, HEI or points? Pix help us see it too.