I cannot figure out what gauge wire to buy for modifying the engine wiring harness. Seems like 8/10/12 are used in different places. Also, a good place to buy in bulk would be a welcome recommendation. I apologize for the really amateur question, but I can't seem to nail this down.

 

I used mostly 18 for the body plugs. I think there is a lot of 18/20 in there, but as long as it's close it should be fine. I know napa sells like 75' of wire for like 8 bucks. But Home Depot or Lowes probably has a good selection too.