That PC power supply is not going to work. it puts out a dirty 12v. you wang an amp on it and itl be toast. everyone is telling you not to get boss and youre still gonna do it. idiot. dont ask for assistance if you ignore all of it
at worst do the big three in 1/0 its like 2 bucks a foot for cca. then do a run of 4gauge to the back. I usually do that when im less than 1k. if you go over 1k id suggest a second battery and at that point youll need 1/0 to the back anyway.