Hate to be a drag, but first of all taking an unloaded cab and just putting some speaker in it is a really bad idea. Cabs are designed for the speakers they have in them, that's just the way it is. You can find a suitable speaker, but you need to reverse engineer the cab first.
Then there's the whole "Mixing cabs" thing you are thinking about. Again, not the best idea. I suggest using the search feature at the top of the page, as there are numerous threads about these things here.
Jimmy M is free. Run.