Rewiring my Duster, but have a few questions

1) I'd run the VR under the dash and pickup power for sensing at the fusebox as that is where the power matters for most things in the car.
2) breakers are fine for fans. I use them on my fans all the time. I don't like self resetting, prefer the push reset. If it trips, having it reset only to trip again means you are shorting out somewhere.
3) I use 12ga for headlight power runs in my kits. Not the place to skimp IMO.

If car is frame connected, drop your neg cable through trunk pan/extension and weld a nut on the rail. Weld a nut on the frame at front. Tie ground cables to the nuts for battery and engine.
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Depending on loads, a # 6 charge wire running length of car may see some decent voltage drop. To minimize that load, run a wire #4-6 to a buss point to run your fans/headlights/ignition and anything else up front. No sense running power to the rear and back up front. Keep all runs as short as possible and your the alternator is the power driver with the engine running.

No cut off switch makes things easier, but you have some long hot runs at all times with battery connected.