Um, "too small"? hardly. You might lose .05cu inches.
You gotta understand - the specs the manuf'er gives you are just general guidelines for an all-around good performance for high and low bass notes. When making sealed enclosures, the concept is to put a sealed pocket of air behind the speaker for it to force against. If you design a box with less airspace, you're simply making it harder for the driver to hit super super low bass notes because you're impeding the ability of the cone to travel in and out as much - the figures the company gives you are just the absolute prime airspace figures. If you want to get technical, your box was probably already smaller than suggested specs of an airspace for the speaker because the cone, magnet and cage of a speaker takes up .15 - .75 cu feet depending on the brand and size of speaker you're using.
Less airspace = "tighter" bass notes and better high bass.
More airspace = more cone movement which helps with thumpy low bass notes
So, you don't need to start over from scratch, just expect that your high and middle bass notes will sound killer and the lower bass notes won't be as punchy/thumpy as they could be. I personally doubt you'll notice - after all, you're not trying to make a license plate rattler
As for your claims that your new radio head unit will have 180 continuous power, I gotta call "BS" on that one. No radio on the face of the planet is equipped to produce that kind of "real" wattage. If it was possible to flawlessly push out that kind of wattage from a tiny car stereo, a reputable 200w amp would be small enough to stick in your ashtray. Head units are not equipped to produce that kind of wattage and any person out there who tells you otherwise is full of poo
. Likewise, if this feat of pushing 180w was really possible, why are home stereos so freaking big? Why do they generate so much heat? It's because of the massive heatsinks and internals that are required to push the wattage you need.
In the past couple of years, we've taken leaps and bounds in working with true digital amplifiers which lets us cram much more wattage into a much smaller package, but I have yet to see this technology really put out in a head unit. If it was, it surely would cost several hundred dollars.
Dollar for dollar, you will not be able to really push a pair of speakers and subwoofers adequately without using an external amplifier.
If you really are dead set on getting new subwoofers, your best bet is to go to
www.google.com and search for "low profile subwoofer" and start comparison shopping amongst your choises.
Don't give up hope. You've done a great job already. I see a couple of people in this thread trying to give you genuine help so you don't fry something and you really get the most out of your stereo with the least amount of money. Nobody ever does anything 100% the first time when it comes to computers or electronics...stereo installs are no different. Take what you can from this experience and use it next time around
I can point you to some install threads and stuff for amps with more info if you want. They're all in this forum.