If you are buying Seymour Duncan, and you have already chosen vintage strat-style single coils, then I'd suggest something that's still vintage and clear sounding.

Measure your bridge string spacing to check, but you'd probably benefit from getting a Trembucker spaced pickup, and I'd suggest a '59 Trembucker as it's a thicker, sound than the single coils, but still has clots of clarity and won't give too big a jump in volume when selected.

You don't want anything too powerful as the drop in volume when you switch to vintage-style single coils will be considerable. A more powerful bridge pickup really requires hotter single coils.

Alternatively you could fit a humbucker-sized P90, so for Seymour Duncan that would be a Phat Cat. That would keep things cleaner still, but still have enough power if you wanted some drive tones. A P90 is a better general lead pickup than a Strat bridge pickup in my opinion.