P-90's are P-90's and Humbuckers are Humbuckers... they're different sizes because they're different pickups.

I see what you want to do... It's going to be a major project. Especially if you haven't done any guitar luthier work. You will have to route out the position that you want to fit the Humbucker... and probably do some routing to accomodate the dog ears... I suggest taking it into a guitar shop that has a tech or a luthier and asking these questions. It's difficult to say how much work would be involved without seeing the specific pickups you have and the pickups you want to install... among a lot of other variables. Good Luck