First off, do you want to play the system when the car is off?
If no, then sell ALL your batteries cept the one under the hood for starting the car.
Spare tire removal.... why? Weight savings when you're going with BTL's? Ha right....
And why in the FVCK would you want to put your amps UNDER anything? They'll just suffocate.
Get them out in the air, if you're serious about BTL's consider some AIR FLOW via fans.
I think you're wasting your money.
BTL's are for comp use only. They sound like total dick when listening to music.
One note will be 10 times louder than all the rest.