Not a poor connection or open connection in this case
Fusible link working correctly = battery grounding.
And yours worked correctly. It melted quickly when the battery was hooked up.
Therefore something on the power circuits grounded.

Start with alternator output stud. If you have an ohmeter, disconnect the output wire from the alternator and check for continuity to ground. Check the stud, check the wire.
If its in the wire, disconnect at the bulhead and check continuity. Keep working your way back.

Without an ohmeter, you could hook up a 12 volt, low amp light. With a device in the line, that should restrict the flow. Another trick is to wire a flasher unit to act as a quick acting, resetting circuit breaker. I think that's in on of th Chrysler master Tech pamphlets. I haven't tried that myself.

Below is an example of a fusible link in action. In this case it shows a wire that has been rubbing against metal until the insulation was worn through.