When I did the second battery in my old 60. I put the same fuse block you have onto the driver side kick panel, next to my left leg. It made for an easy and clean install. I had it inside for all my radios, 12v plugs etc. It was easy to run wires through the rubber grommets in the firewall into the cab. Under the hood get's pretty dirty, but if your fuse block seals well, might be ok.
'73 FJ40 - '85 FJ60 - '05 Tundra