[AceX]

If you ditched the center section, those rails look like they would provide some real nice support for a box.

Well, with the tank behind the seat, and basically a lowered bench, your options are very limited.

You could try and fit a single, or twin Pioneer 8" shallow mount subs under the dash. Matched with an Arc Audio KS125.4 Mini amp to power everything will produce incredible sound. Only hard part will be figuring out the enclosure, which will probably have to be made out of fiberglass.