[RichardJ]

Yes, the two fuses are for the Ammeter so don't mess with them. Your diagram isn't complete all the way way to the dash or you would probably have found where they went. If one or both of the fuses burn out, the gauge will not work.

That is not the best place to pick up your 12 Volts. The red wire that goes from there back over to the battery is already serving as the Alternator charging wire and provides the shunt for the ammeter. It doesn't need you tying accessories onto it.
I would recommend replacing that single pin insulated junction next to the battery. That one is to small and probably ready to crumble into 40 year old dust. That is where you should tie in your new accessories, next to the battery.

A lot of guys replace the original insulated post with one from a S10 and that is very similar to what you will find in a flaps. If you have a lot of accessories you could use two S10 posts. One would hold the battery wire and all original wires. A jumper from there to a second S10 post that would hold all of the new accessories.

I found a small fuse box with a large post on the bottom to hold all of the original wires. The fuses feed the various accessories as well as some relays in the corner.

In case you were wondering the radiator over-flow bottle is from a '70s Nova.