Equus gauge and voltage issues LS Swap

[00silv4.8]

Hey guys new to this forum but Ive already gotten tons of information via google searches. Just seeing if someone can help me out as I havent found the answer yet:

2 issues: 1- Equus electronic water temp gauge/sender (6262). the gauge is wired to the pass side head (5.3 LS swap) and its incredibly inaccurate using the wire from the factory water temp on my 80 crew cab. I believe my cooling system is just about completely bled and using a laser temp gauge Im getting about 160-190 on my factory temp sender and the T-stat as well as water pump. Heads are showing about 200* ish. Radiator is usually around 100-190 all the way up and down the tank, however gauge is reading 240-280*. Anyone know if it needs a resistor or anything?

Second issue is Im getting a lot of voltage fluctuation. I am between 10-14v via gauge and when I probed my alternator it was at 14 at idle initially after the swap. My radio turns on and off as well. I have the alternator set up on technically a 2 wire, being that I used the red off the factory alt set up and then cut the red wire out of the plug on the alt and put that on the post as well. Now I have the black wire that is hot, key on, but I have it capped and taped for the time being as Im not sure what its intended for, I did find a diagram saying its for amp indicator light, but isnt necessary on the LS swap? Also have the brown wire from factory engine harness in alternator plug.

Any help is greatly appreciated sorry it was long.

Digest version:
80 crew cab, LS swap with 5.3, fresh rebuild. Factory harness and fuse box.
1 issue Elec water temp gauge (Equus 6262) incredibly inaccurate
2 issue Volt fluctuation, radio turns off, gauge shows 10-14v. 2-wires on back of alt which go to the starter (how my truck was when I took ownership with big block) and to fuse block, engine bay side.