you have a dead short somewhere or your fusible link is not rated for enough amps.

Start by pulling all the fuses, disconnect the large cable off the alt and insulat it so it doesn't touch anything.

Then put the fuses back one at a time till it pops again. see what circuit it is and figure out where your short is.

IF it still blows instantly, then unplug the headlight switch and horns.
Not sure what else always has power, but start looking at your wiring inside and the bulkhead connector for a short.
Might even bypass the amp meter just incase.