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Well honey looks like I will need to buy a new engine wire harness for the car. (Wife) how much does that cost. Only 700 dollars...
LectricLimited has your engine harness for $304 - and it's not that it makes the car run better, it's that it makes it safer, more reliable, and not a puddle of melted glass on the side of the road. My '72 harness looked much like yours with splices, broken connectors, and stiff, cracked insulation. I'm so glad i changed it out - it's simple to do and fixes so many unknowns.
To add to what 4-Vettes commented, my '72's ground cable is connected to the underside of the passenger engine mount horn and the side of the block where the forward starter brace mounts; it's a short cable only a foot or so long. I'd bet your '73 is the same configuration.
Here are two of Mooser's pics illustrating the engine ground:
