Yeah, I ended up finding some 4Ga welding wire (red and black) long enough to do the 3 cables I need to do. Forgot I bought those 3-4 years ago because I already didn’t like the thicker cables I made before then and wanted to change them out. The stock cables are 4 gauge to starter, 4 gauge to engine block ground, and 8 gauge to the firewall ground, so I guess the firewall will get an “upgrade.” They all still fit in the small section of loom at the battery tray so it’ll work.That's what I did, though I started with an already intact harness. Took it down to my local Orileys and one of the more helpful counter guys spent 20 mins with me getting the materials together and building all the battery cables from scratch. Think it was about 60 bucks total.
I need 2 more ring terminal lugs in 4 gauge (for the two grounds) and then I can terminate and hook them up. I have all the parts for everything else and I’ll cover it all in loom and a bit of tape when I’m closer to done. I’ll share how it turned out when done. It will basically be identical to stock except for being welding wire, that one upsized wire, extra loom vs stock, extra tape vs stock, and military terminals instead of stock.