Old g/f of mine had a '63 vert.....with a rusted out fuse block, and I have seen others including my '72 vert with same troubles....The fuses themselves tend to melt the solder between the element and the metal caps, due to the steel clips on the block/holder being corroded, and then they were never soldered to the copper bus plates, just crimped over, I have seen those crimps give up the ghost and start getting all burnt and dark looking, dropping voltage of course, and causing fuse failures as above....
the remedy seems apparent, clean and solder all connections including where the wires are fastened down...polish up the steel clips....
In all the cars I have had over the years, I have never bought a new harness for any of them finding plenty of length in the old wiring to cut/strip/tin the wire ends and solder down....course I don't mind rerouting at all either....like in my '72 here, only section of that firewall plug that is left is the front lighting, all the rest is disabled, and wires run up higher near the gas pedal cable, and making it easier to work with, no plug at all, I had one of them mess up BIG TIME decades ago, I pulled out clip leads and a jumper cable and drove the car home that way....never did act up again, nor could I find what failed but it was in that damn firewall plug somewhere....:crutches: