P-90's are narrower than humbuckers, they are the same size as the mini-humbuckers that Gibson uses in the Les Paul Deluxe. If you want to add a P-90 pup to a full size bucker rout, Rio Grande, Harmonic Design, and a couple other companies make 'em.
you can get a p-94. i don't know how close it really is to a real p-90 though. it's supposedly just a humbucker sized p-90. i don't have any personal experience with them though. the p94 is a gibson pickup, i'm sure other companies make them too.
Lindy Fralin has a boutique pickup company that makes hands down the best P-90's available. I've played a 50's Les Paul with the original P-90's and a new Gibson with Lindy's P-90's and they were almost identical (and my guess is that the guitar itself was the main reason for any discrepancy). Honestly, he's incredible. You can also email him and ask if his P-90's will drop right into your specific guitar.
