Yes ; Welcome to the Forum Igor ! :)
I have not used my Manhattans in the context you want, but will say ( as Kiichi just mentioned ) - that the Mississipi Queens are quite powerful , in a clear but sweet way ; and so I would only use a Manhattan in the neck position with another 'Vintage' pickup in the bridge, and not a Manhattan in the bridge - unless it's in a Manhattan Pair.
Like you , I absolutely love the Alnico III sound - and I find that the way Manhattans are wound , really make that choice of magnet richer, fuller and more rounded, without losing any of that open character. Manhattans don't 'push' - rather they 'Bloom' wonderfully under the pick.
My vote would be for Manhattan Neck ( Heaven ! ) with Mule bridge , or P.G Blues. The Stormy Mondays are lovely also, but for the bridge position , I think a more focused magnet would be the appropriate contrast. This should give about the same percieved output across the pair, with a neck pickup that absolutely seduces - and a bridge pickup that takes charge of business, without betraying the tonal quality of the neck.
It's is however, speculation on my part ; as my Mules are in a Les Paul Standard - and my Manhattans in an ES-175 copy , and neither have been taken up to the level you will be using - BUT their characters at low volume and 'clean' certainly give me the impression that that combo would work very nicely.
As to " How close do they sound to a real P90 " - I gather from the other posters on here , that the Mississipi Queens are fatter with less of a dry 'cut' than many P90s, _ and certainly, the Manhattans sound postitively Luxurious ; but now B.K.P. do a much wider range of P90s and HSP90 - that generalisation may have changed.
As Kiichi also implied, there are now even ones that might rival the Manhattan - so if in doubt, ask the very helpful players and advisors at B.K.P. Meanwhile, Have fun !
