You should've read the rules that YOU AGREED TO. "what would be louder" threads are against the rules. Wanna know what will be louder? Build both systems and tell us.
I understand completely. But nothing is better than personal experience. If I were local to you to help you, I would. But the best thing for you to possibly do is just go for it. You've already been given advice. Now just go get her built.
I would pre drill before you do any glueing. And do you know what predrilling is? Find a drill bit a little smaller than the screws you're using. Drill with those. Then use the screws.
Nice system choice. I'm running 2 mayhem 12's on an aq3500.1.
For welding cable check out weldingsupply.com, or check out seller name "coppercableman" on eBay.
I always suggest doing a run of negative from front to rear. But if you can find a really good ground in the back. Then you're good. But on my car, I have a batt ground going to the engine block, then a ground run from front to back.
I would do one run of positive from the front batt to the back. One run of ground wire from the front batt to the back. And one run of positive from the alt to the back.