Ok here are some pictures. They are not of a 1961 Chevy because I have never owned one. Over twenty '55 Chevys and a '53 but never a full size. My 1961 was a Corvair Monza Convertible four speed car. Then came a brand new off the show room 1967 Chevy II two door followed by a 1968 Camaro and a 1971 Nova.
Anyway it is a GM product and all GM's share parts and technology (the SBC was designed by the man that built the V8 for Cadillac that was so popular ion the fifties between a set or 1928 model A frame rails with a '32 Ford body on top.
The Fuse block inside the car cabin looks like this:
This is a picture from American Auto Wire and is as close to a factory harness as you can buy today. Note that all of the wires are bundled together with plastic wrap (not electrical tape) and that the only wire to come off the front of the fuse block is the double red going to a power tap connector. There are no other wires tapping into the front of the fuse block from the factory. Anything you see there now was installed by a previous user who may not have had your skill level/.
Next I direct your attention to the labeling. TAIL is for the tail lights, not the brake lights. It also powers all of the marker lights on the car (starting in 1968 this included side maker lights). STOP powers only the stop lights, whether you have two four or six bulbs it is all powered here by way of a brake light switch under the dash. EMR powers the emergency flasher which became standard equipment in 1967, so it would be a dealer installed option on your car if it has a chromed knob with a garnet red plastic lens in the center of it mounted just under the dash.
The two red wires I mentioned above are plugged into the PWR spade. That terminal is live only with the IGN powered up. It is were you tap something you want to be powered with the ignition switch in the run position. The separate ACCS spade provides a power tap when the ignition key is turned to ACCS or in the run position and is good for powering a radio that you can listen to with the motor off.
GAUGE and INST LPS power the lamps for your dash and the idiot lights get their power from the Gauge fuse. If you have meters (Amp, Tach, Oil, and Temp) that requires a piggy back supplemental wiring harness that plugs into the cabin wiring harness under the dash. But the power is supplied through the gauge fuse.
DIR SGNL and BCK UP power your turn signal lamps and back up lights. Two separate switches control those lamps that share one fuse. The Radio has it's own circuit and is the tinny yellow wire coming out of the back of the radio which back then had vacuum tubes and was generally only AM.
HEATER powers the fan switch which drops the power to ground through a series of resistors to control the fan speed. WIPER runs straight to the wiper switch was only one speed back in 1961.
IF your car had Air Conditioning (a rare option) it would power the compressor through the AC fuse. The BAT is a direct connection to the battery (through the front wiring harness fusible link on the fire wall as well as the main fusible link at the battery cable terminal). It is not fused individually and if shorted will blow open both fusible links in your car's wiring harness.
Here is a picture of the full factory car wiring harness (out of a 1967 Buick Electra 225 I scrapped just to obtain the Buick Wild Cat 430 engine) which as you can see is bundled into branches like a tree to route power where needed in the car. There are no loose wires.
By loose I mean not leading back into a bundle that terminates inside the fuse block.