DC/DC converter 12V > 5V providing 400 Watts?
400W @ 5V is 80A. I'm pretty sure the wiring in your "LED strip" is not capable of handling this, so I'm guessing that you're actually connecting multiple strips together. Do yourself a favor and provide separate regulators and separate power wiring for smaller groups of strips. Perhaps 16 groups of 5A each. The input current to each regulator would be about 2.5A (if the converters are about 80% efficient), which can be handled by reasonably-sized wires with relatively little voltage drop.

Car batteries usually have capacities on the order of 40-60 Ah, or about 480-720 Wh. If you want to be able to start the car, you should use no more than about half of this capacity, maybe 250 Wh tops. This means that your could power your display for maybe about a half an hour, after you take the power conversion efficiency into account.