PO converted 1982 6.2 k10 to gas....with wiring problems

[Danvis]

Ill come back to this thread later becuase one day it may get rewired. But for now any help is appreciated. May be lengthy but gonna try to be thorough.

Fuel gauge doesnt work. All of the following led me to find this plug coming from the selector switch not plugged into anything. Normally id plug it in and see if it fixes my problem but wanted to see what yall thought. Not knowing if it would even be a reason for a my fuel gauge to not work and being unsure if it was unplugged when it was changed from diesel to gas for a good reason or not but want to be sure im not gonna pump diesel to my motor because i dont know if the other tank was swapped or only one tank....if it wont fix my gas gauge what is it for.

Cleaned ground and frame by the tank. Checked the printed circut for the gauges. Should be a new..ish sending unit so i havent went through the trouble of dropping the tank yet......last resort because dont know the conditions of the passenger side tank so at some point id like to change both tanks and sending units and atleast the lines from the 2nd tank unless i dont understand how its plumbed correctly i dont know how the lines from that tank to the selector valve are either.

Also any help with what wiring harness i should get im a little unsure...i read alot of good about the painless and for something like wiring that i want to last a long time the price isnt completely insane. But the truck was originally a 1982 K10 6.2 diesel with an auto trans. The guy before the guy i got the truck from swapped it to a 1982 k10 with 350/sm465-np205 with a 10 bolt 8 lug front and what i was told was a newer e350 srw D60 rear. Is the harness from painless designed for auto or manual as far as neutral safety, reverse lights. Not a have to have for myself but if im going to go through the trouble of rewireing id like to do it all not parts