Looks good and a rather extensive install, props for taking it on.
For the wires under the car: use a buttload of wire ties as you do NOT want the power wires to rub on anything. For spots you can't tie down that might rub, cover them with heater hose.
I've been doing trunk mounted batteries for street race cars for 20+ years with the power wire run under the car and have NEVER had a problem. Wouldn't hurt to inspect things underneath every few months just to be safe.
Good luck on getting it finished, should be loud when it's all done....