Indeed, conventional wisdom says put a P90 in the neck position if you're only going to use one P90. It does sound great there, but being a fan of the Telecaster/Les Paul Jr/Melody Maker twang, I really like the sound of a single coil in the bridge position. So I'd recommend swapping both pickups to P90s. Sumitomo might be on to something there, I've never tried P-rails pickups, but I've heard good things about them, so they might be a safer bet.
That being said, if your guitar can only suit humbucker-sized P90s, here's part of an email I got from Jason Lollar when I asked him about humbucker-sized P90s a while ago:
"Part of the tone of the P-90 is that it has a low wide coil. The coil is approximately ½" wide on each side of the pole pieces.
On a humbucker you only have about ¼" between the pole piece and the end of the pickup cover- a P-90 coil extends ½" beyond the end of each pole piece - the high e and the low e.
In order to make a single coil that fits you have to make the coil taller or wind it with a smaller diameter wire - (which is what I do) to keep the coil as low and wide as possible.
You loose midrange without the wide coil so all of these pickups have less midrange than a real P-90 to varying degrees. A tall coil will sound brighter than a lower wide coil too."
Not sure how keen you are to get the "original" P90 sound, but there you go. Hope this helps.