Had headlights, then high beam only, now nothing

The link is getting warm because the circuit is overloaded.
Do not continue to put 35, 40, or more amps through the circuit.

You will be lucky if only the fusible link is damaged.

The fusible links are usually 16 gage, but I don't know '71.
Chrysler used blue insulation for 16 gage link but others use different colors. The gage is more important than the color.
I think you can buy premade with the correct connector and terminal.
You can certainly buy fusible link from most automotive parts places etc.
Your link is prob attached into the bulkhead connector or the starter relay. Depending on which will determine whether it needs a ring terminal or a female spade type with barb (Packard 58)

Please go back to post 22 # 27. Print that schematic if needed. Ask any questions if it doesn't make sense.

If I understand correctly, the ammeter pegged discharge with the engine off, headlight light switch on, and neither headlight bulb installed.
Is this correct ?

If not, then it would be really helpful know what voltages you meaured at the alternator output stud with the engine off and running lights off.
I have checked the alternator output & monitored the battery voltage, when I turned the lights on
What were voltages?

So far the only things we have to go on suggest the circuit breaker is tripping and the fusible link is getting hot.
This points toward a short in the headlight wiring after the circuit breaker.

It can be either traced visually or with some equipment (test light, continuity meter)
People here can walk you through this. But we have to follow the clues and test the possible reasons one step at a time.