Fusable link questions

73 duster 340 was a slant6 car with column [ auto] i coverted to floor [auto] steering column is from a floor shift car 73 also. the key switch works some times but most of the time i jump the soliniod the amp meter sometimes goes to H when i turn on the fans it goes just past the middle so what causes the fuseable link to get so hot but not blow

Voltage regulator or a break in one of the wires to or from it. If not then in the alternator it is shorting/grounding.