Why are you so dead set on taking a perfectly working cab and making it into something completely different?
A better bet would be to sell your current cab and use the proceeds to build the cab you want from scratch. Or at least find an already gutted and/or blown speaker cab and convert that instead.
You're unlikely to find a single 12 that is going to actually handle the 400w you are aiming for. Even 2 cheap 10s will probably outdo most single 12s, unless you spend a ton of money on the 12.
You also didn't mention what kind of cab this is to begin with, which will also influence what you can even do with it and what kind of advice you will get. Not to mention, the dimensions you give sound extremely small, unless this is a very deep cab (seems unlikely because they would probably just make each dimension a bit bigger then), so the performance of any speaker you put in it will be limited by the internal (spatial) volume.
But because it sounds like this started life as a cheap budget cab that is likely too small to do anything serious with, I would recommend selling it to someone who will use it as is, and using the proceeds to build something from scratch. Since it's likely the cab isn't worth very much, you could probably go ahead and keep it until the new homemade cab is ready to go (unless your budget is really that tight in which case you might have to go without a cab for a while, if you don't have another to fall back on).