The way I did mine was to use fender washers on the floor above the gas tank. I believe I only have 2 or 3 of the available holes in the bottom of the summit box actually bolted to the floor on the passenger side. On the drivers side, mine sticks into the spare tire well a few inches so I used some strong angle iron and bolted one side to the bottom of the box and the other side goes toward the passenger side of the spare tire well using some long thick bushings. It isn't welded or anything, but I took my time doing it and I can actually slide the tire well cover under my box to keep the floor area rigid. You could pick my car up from the battery box. It is plenty strong with large fender washers and attention to detail. I had to drop the gas tank to do this but you'd have to for practically any install I believe. Point is, make it solid so if the track officials yank up on the box, the whole car moves and the box don't at all. One of the techs at my track smirked when he tried to pull up on my box and said, "yeah, that's solid".
As for wiring, I put my starter solenoid in the box with the battery, along with a Flaming River cutoff switch. I only used 8 gauge wire running to a mounted stud on the fender in the front. A 4 gauge wire goes from here to the alternator. This is plenty big. Ford only used the equivelant of an 8 gauge wire to connect the alternator to the positive side of the solenoid. I used 8 gauge runnig back to the battery from the mounting stud in front. This is only used to keep the battery charge when the car is running, or run some accessories while the ignition is off. It easily handles my Mark 8 fan and headlights and radio. Those with aftermarket amps drawing a lot of current and fog lights may want to go bigger.
For the relay, you want large wire (preferrebly welding cable 1/0) running to the relay and from the other side to the starter.
This is carrying all the current when trying to start the car. If you go too small, you run the risk of poor performance when the car is hot and the cable is hot. The battery cable may come close to your headers or exhaust system. This will cause the cable to run hot and it will reduce its current carrying capacity. Plus it is about a 12 foot run depending on how you route the wire.
Welding cable is strong and very flexible. It is composed of many fine strands which make it both flexible and excellent for carrying large amounts of current.
**Use large welding cable and you won't every have to do it again**
JCM3 has a good idea with the bolts being welded on both sides and using one as a ground. I ran a 1/0 wire out of my box right to the spare tire well wall and used a couple washers to secure it. I tested the resistance from this point to the front of the car at multiple spots with practically zero resistance, so it makes for an excellent ground. Better than the factory ground even. My car cranks the same in cool weather as it does after multiple starts when it's hot. Proof that large quality cable and a good ground is priceless.
As far as making the system track legal, do a search, there are a few article with wiring diagrams and excellent instructions on how to to a complete job.
The thing that is nice about having the solenoid in the back is that there is a very short run from the positive battery terminal to the relay, and this is contained within the box in my case. This wire is hot all the time, but again, only about 9 inches inside the box with professional grade terminals, crimped and heat shrunk. The positive wire going from the solenoid in the box to the starter is only hot when cranking.
If you want some excellent advice in person, call Mark at MAD Enterprises http://www.madelectrical.com/
and he will tell you everything you ever wanted to know about automotive electrical wiring and battery box installs. He has been feature in 5.0 and Muscle Mustangs and Fast Fords, plus he's as nice a guy as they come.
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