Or you could send the P90 back and exchange it for a P90 in a humbucker size. There are LOTS of those around. It would be a lot easier than most of the mods you can use to accommodate the P90, and would sound very close to the same.

Generally you want the louder pickup in the bridge position. Ohms is not a perfect test of that, but generally a P90 will have a bit less output than a humbucker. Set at the same height, with two pickups that are the same, the one at the neck will be louder. That is because there is more string vibration which produces the output closet to the neck. But you may be able to compensate for this with string height. The high tones are not as pronounced at the neck, and P90s are better at producing clear highs than most humbuckers, which makes the position Simon suggests pretty common.