Power supply question in regards to powering 400 LEDs
In theory there is no problem with your plan, the strip won't be damaged as long as you apply 5V to it. But if we assume 30A load at 5V and 90% efficiency (which is not certain) than you will draw 150W from the buck and about 166W from the battery.At 12V this means about 14A from the battery. An average sized car battery with full charge would be empty in 5-6 hours. And as mentioned in the comments you have to use big wires, otherwise it gets worse.
For me a better idea is to buy some 12V LED strip, it would be cheaper and easier than making or buying a 40A buck.