Joined: Jan 2010
Posts: 9,835
Likes: 1,816
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
!26mm is enough for any proper 6s cassette hub.
124mm is normally fine as well (if a couple of 1mm-thick washers are removed from the axle assembly), and the clearance will only be missed if some axle-stop or claw-hanger hardware happens to be present on the driveside dropout.
Suntour cassette hubs should not be built into wheels imo, since the cassettes are so hard to find at decent prices. Shimano cassette hubs can be harvested from bent rear wheels and the Shimano cassettes can even be re-spaced using sandpaper on a few of the plastic spacers.