You are installing a new wire harness with it's own fuse box. Wires are routed from the fuses to various components. Different wires have different fuses and are connected inside the fuse box to accomplish a safe and proper electrical system. Some wires from the fuse box are "switched hot" and some are "always hot". The ignition switch turns on the hot fuse bank. Some wires are direct from the battery and etc. I don't know, and you don't know, exactly how all the circuits are internally wired and engineered. I have spelled out what the instructions say. If you don't want to follow them, just splice, dice and jumper as you see fit.
"Green wire from tach". "Didn't use the 121". "red power wire of my tach connected to 39" I'm not sure what you are doing. The HEI's I've used do not have their own wires. I assumed a GM HEI. I can't begin to figure out what you are doing. It's too risky for me to advise other than follow the instructions. You see what the instructions say, your ability to wire is low to medium, so follow the instructions or ad lib.
"I could still see someone using both wires though, just bc "the instructions said so".... This shows that you don't wish to follow the instructions for some reason.
The tach needs switched power.
The gauges need switched power.
1. 3A Pink goes to Positive terminal on HEI
2. 121 White goes to tach terminal on HEI
3. Gauge power, including gas and volt are supplied by 39 Pink. When ready, I will advise this hook up.
