A P-90 is longer (pole to pole) than a humbucker. A humbucker is wider (neck to bridge) than a P-90. They also mount differently. I don't see it working on a Paul, you will have some ugly exposed wood there.
It wont fit bro!!!! If you want a P-90 in a Paul, just get the Phat cats! Dont make your LP look like Macgyver installed a set of P-90's in there with some kind of hack job!