I used to work on both big trucks and generators. Man what a joke. Open the panel of a generator to find a bundle of wires as thick as a beer can, ALL WHITE!
With them being all the same color, it makes it a royal PITA to trace unless they have legible wire tags running all along them. Then open up some of the big truck dashes and same thing but ALL BLACK.
If you only use 2 or 4 colors, you will be in for a world of hurt.
If you use generic wire in 10ft roll from autozone you may also be in big trouble. The wire is cheaply made and has a cheap non high heat resistant insulation. Not being as oil/fuel resistant either.
You want high grade automotive wire such as TXL or marine grade tinned wire. Not old scrap you salvaged from a '76 ford.
There are atleast 20 places that the factory wiring needs improved on. Every single switching mechanism in there has a design flaw
- blower switch
- ignition switch
- headlight switch
None of the gauges are anywhere close to accurate, all are merely suggestions. Speedo is the most accurate and it's crap. The fuel gauge is more of a "yes or no", temp gauge is crap, oil psi is crap, amp gauge is crap (I think volt gauges didnt come out til a year or 2 later)
Headlights are quite a bit dimmer than they should be due to high voltage drop across the wires/switch.
Luckily a cummins truck doesn't need many wires to run and operate. Then you can work on cabin creature comforts one at a time, along with lighting system and you are done.
I would tell you to get a wiring diagram, but then again, I'm not sure how good that will do you, since between the engine swap/correcting factory deficiencies/gauge/lighting upgrades, the end schematic would be completely different from any factory service manual.
Best to try to break each thing down one at a time.