Sub Box? Help me please

[78' Silverado]

Is there a specific amount of space a 12 inch sub needs to get the max amount of bass? I got 2 12's and a 600 watt amp and I hit like there 6 1/2's lol. 2 12's should be hitting hard. the way mine is setup is like so. The length of the box is the lengght of space behind the bench seat ok. Theres 5 sections to this box

1. On each side I have 1 6x9

2. After them I had the 2 12s

3 then the amp in the middle

So it's l--6x9--/12' sub//AMP//12'sub/--6x9--l

Each speaker has its own enslosed "box" now....it is not sealed in the inside but it is sealed on the outside. Given the amount of lengtth i have to work with....i figure I didnt have a big enough compartment for my subs to get the max bass. does any have specs on wat will be good for a 12 inch sub. The amp can be moved to a different place...and the 6x9s I am trying to come up with a way to make kicker panels to hang above my head in the 2 back corners...;..still workin on that one :/ lol. So any info on the pace needed for a 12 inch sub would be great guys. If there isnt any....and i just made a crappy box....let me kno HAHAH :P Thx again guys!!!

Tyler