If you want it routed properly for a P-90, you'll need to phone up (though they may also do it via e-mail) Warmoth and make the order that way. That's if they'll even do it; they seem to get a bit iffy when it comes to doing things with the less common bodies.
A far simpler and more flexible way would be to simply use standard-sized humbuckers, and use a humbucker-size P-90, such as a Seymour Duncan Phat Cat. (There are plenty of other variations, that one is just the most common.)
I've done the 'Tele Deluxe that sounds like a Gibson' (and a couple of Jazzmasters that sound like Gibsons) several times myself. I've swapped necks between them and tried all sorts of wood, hardware, and pickup combinations, and I've got a lot of Gibsons to compare them to, and I'm pretty sure I've settled on what you need to make it work:
- Mahogany body and neck. The thinner body and bolt-on construction makes up for the lack of a maple cap.
- Rosewood fretboard. The guitar will already sound lighter and brighter than any Gibson due to the construction, so you don't want to push it even further away by using maple, ebony, etc.
- 24.75" scale conversion neck, though I've also tried a 25" scale conversion neck and that sounded more-or-less the same considering the woods involved. However...
- ...Not a Warmoth conversion neck, because those only come with the 'pro' construction double truss rod, which expands the high and low tones a lot. Almost double the metal mass inside the neck runs counter to the Gibson tone. Other parts companies make conversion necks which do not have the double-heavy truss rod and will retain the tone you expect.
- The thickest neck profile you can play well on will help bring through the Gibson-esque sustain and resonance.
- Either a tiltback headstock or really severe string retainers on the headstock to ensure the strings get the proper break angle over the nut. Too shallow an angle will make the strings sound more 'plinky', for lack of a proper term.
- 300k pots on at least the volume controls. Gibson actually use 300k pots for all controls on their historic/reissue guitars (and 300k is what was actually used in the 50s) and 500k pots on the modern guitars. It's really up to you which you prefer, but I've found that 300k volume and 500k tone is the best 'average'.
- Stick to A2 or A4 pickups. A3 and A5 are common in Gibsons, but in a Fender-style guitar they become much brighter. A2 is your safest bet, with A4 being the furthest you should go if you want a lighter tone; A5 will come out much, much brighter than any Gibson tone.
For reference, one of my 'Gibson-likes' follows all of the above, using SD Phat Cat pickups, and while playing there is no discernible difference in sound between it and one of my Les Pauls with 'full' P-90s.
If you're not adverse to a bit of pop-rock, for a while two guitarists from Paramore were using Fenders which had been designed to mimic Gibson sounds (a Tele Deluxe and a Jazzmaster) and they did so using Seymour Duncan Alnico II pro bridge pickups and Phat Cat neck pickups. They stuck with 25.5" scale, but they also used 250k volume and tone pots to smooth out the high-end to compensate. I've tried it myself and I find it's fine for rhythm but too muted for leads, and I prefer going with a Gibson scale and a 300/500 pot combination, instead. Still, that pickup combination definitely works.
I've also had great success using Gibson Burstbucker #3 humbuckers for the bridge with a bridge model Phat Cat in the neck. The Burstbucker #3 is overwound and thicker-sounding than most PAF copies, and using a bridge Phat Cat in the neck also adds more weight to it; great for compensating for the brighter and lighter tone of a Fender-style guitar.