Moving the battery behind the passenger seat is not a bad idea. You would lose pounds of cable doing so. The battery box can be plastic but the battery itself has to be bolted down with metal to the body. I have also used a Deka 16 lightweight AGM battery on my old 340 before and it started up fine but had issues on hot days.

Aluminum blocks are available for the Gen 3 Hemi but it is another $4 grand extra