There is a few things I want you to do if you're relocating!.
1) INSTALL AN EASY TO ACCESS KILL SWITCH FROM THE GROUND.
2) INSTALL A KILL SWITCH FROM THE GROUND
3) DONT FORGET THE KILL SWITCH FROM THE GROUND.
the number of people who relocate batteries and don't put a kill switch on for safety is just beyond me. If I do my relocation, I'm installing a special pull wire which will be inside the full filler cap which when pulled kills the battery. The Reason? If you have a crash, or something is going bad, it's easy to kill the battery.
Running it in the car is fine, but it needs to be vented atmospherically, so a simple breather pipe to the outside of the car is fine. I've seen many car Audio places put batteries in and run a 1/4" clear pipe outside the car from the box! It's legal and easy.