installing new battery cables on '66 barracuda

The original system has the main battery cable going to the starter and the #10 going to the starter relay Batt terminal. The extra wires you have, may be additions by prior owners. By using an ammeter it is possible to identify loads that may drain the battery while parked.

Things to check for getting the alternator going would be: voltage regulator, its connections including ground, bulk head connectors, amp gauge connections. There are schematics at MyMopar.com that will help identify wiring circuits. You will find them as Figure 15 in section 8-117 and figure 16 section 8-118. I have found the information to be accurate for my 1966 Barracuda.