A lot of the classic jazz boxes have actually had pretty clear sounding pickups in them. All those classic Gibsons with P90s or PAFs. So you'd really want something low powered with clarity, but not as thin sounding as a Strat pickup. The mellowness comes from the use of the guitar's and amp's tone controls and having the pickup as near to the neck as you can. The more powerful the pickup, generally the higher the level of mids and the less treble you get, which means that the sound gets more muddy than mellow when you start turning down the treble. So for a humbucker, something that's close to an original PAF, with Alnico 2 magnets (for lower power rather than the more powerful Alnico 4 or 5 magnets also used in PAFs - they varied a lot) and nothing overwound. So yes, the Tonerider AC2 or Iron Gear Blues Engine. Seymour Duncan Jazz (a very clear sounding PAF style neck humbucker). The standard Bensons on the PBG site look a bit hot to me, though you can get one made to your own specs.

And flatwound strings are a must.