I would choose a vintage output classic stack pickup for middle position.
1) The Seths are A2 magnets, so the output will not be too much to cause a mismatch. Classic stacks give more output that true single coils anyway.
2) With a hot middle pickup, the extra mids can make the notch positions indistinct / muddy. Tone-wise I prefer a brighter tone in notch positions, but you might prefer something else.
I would go for the middle version of classic stack plus. The middle version has *South* magnetic polarity. It can potentially do hum-cancelling in pos 2 and 4 if you decide to use coil-splits to the slug coil of the humbuckers.