Good day, I have an 01 GT stroked, procharged... I ran out of space to install my oil filter relocation/oil cooler thermostat, so I need to put the battery in the trunk. I like your approach. Did you happen to write out a wiring diagram by chance???Just keep it away from anything moving and anything hot. General rule of thumb is at least half a foot from anything hot and in that case the wire/hose/etc should be covered in some type of heat resistant sheath like those fiberglass woven spark plug protectors. I might even go father and cover it in that as well as the corrugated seamless aluminized tubing.
More than one way to slice the pie, and there's tons of examples in google images or mustang forums, etc. Did the trunk relo a few years back and while I'll post pics for examples, I honestly threw money at it and it's not for everyone on a budget. I have + running down driver side and - down passenger. All 1/0 AWG or 0 gauge or 50mm squared.
It's a Varley red top 60 with a custom aluminum hold down (pictured), 500A main breaker (pictured), a + distribution point up front (not pictured) with one leg going unfused to the starter and the other leg going to everything else (300A fused toward OEM fuse box and the alternator). Negative trunk goes from the IRS subframe onto a distribution point up front (not pictured) and branches off with one to the passenger motor mount and to the case of the alternator.