Also, I have the Alpine M450. 220 RMS @ 4-ohm or 400 RMS @ 2-ohm. Should I go with a 2-ohm sub to get that extra power? Are there considerations to look at for 2-ohm v. 4-ohm?

The amp will heat up more with a 2 ohm load. Also, the lower the ohms, the lower the damping factor (the amplifier's ability to control the woofer). Try it at 4 ohms first, then 2 ohms and listen for the difference. If the volume at 4 ohms is good enough, stick with it.

edit: nevermind. I just reread your post. You don't have the subs yet.

 

You can screw pieces of 2x4 to the inside of the box to reduce volume. It's simple math to figure out the size you'll need. Do not add polyfill or any other kind of fill because that increases effective volume. Frankly I don't think .1 cu.ft. is going to make a huge difference. That said, there are subs which fit this box better. I have a Diamond Audio D6. You can find a dual 4 ohm D6 for about $150. eD has subs that fit .75 cu.ft. as well and they're also well under $200.

 

In a sealed box - increasing the box volume beyond the "optimum" tuning (usually a Qtc of .7) will slightly deepen the bass response and slightly reduce power handling. The change will be insignificant. Don't forget that the driver itself will take up some internal volume. You can take a crude estimate by measuring the magnet diameter and thickness. Calculate the volume of a cylinder. If your box volume is raw (total volume), putting the driver in it will probably take close to 0.1 cu. ft. The driver frame and the depth of the cone will use up a bit more volume. So you should be very close.

 

If you like JL Audio, here's one that might fit your specs. Its only $159.00

http://www.woofersetc.com/index.cfm?fuseaction=product.display&Product_ID=2895