Yeah, watch the power when you go big or you won't know when it is too late thermally and mechanically. XBL sneaks up on you if you haven't heard it before.
bigger size doesn't equate to more xmax. Turn them into 10s. You obviously want good sound. I'd rather have all the room for a good designed box than trying to stuff 12s to "fit".
I have no 2.5" spiders or cones. All 3" except maybe the 8" cone which is a 1.5" or 2". How many cones and dust caps, matching or not? Do you want just all of them? No coils.