Originally Posted by GrannyGear
My mistake above.....thought he still had the original hub. Doubt he'll find any 6spd HG cassettes. Might be time to retire the 6spd index shifters and go to more cogs. I assume he still has a 126 rear. Sheldon's selling NOS 7spd 105 hubs (mentioned above somewhere) that will take UG (6spd) or HG 7spd.....or an easily altered 9spd cassette (leave off cog and spacer).
Should have questioned OP for more info before making suggestions. Oh well.
The 105 hubs I mentioned above have the threads on them for the locking cog, and, the cassette you linked to on LooseScrews can be used on a HG cassette hub (it says "This uniglide cassette fits all Shimano 6 speed cassette hubs EXCEPT FOR Dura Ace hubs). So, you have solved the other part of the equation. Good catch ;-)
There is one other Shimano 6spd UG cassette, but it is a 12-34. That would likely need a change in RD to account for the 34tooth cog.
Those 126mm 105 hubs can also be obtained from Loosescrews and Peter White, but Harris has the best price.
Edit: Ok, he already had a new wheelset with new hubs. The hub body should be able to be switched to the old style with the threaded edges. I am pretty sure these can be had from Harris. It would be a Hyperglide(Uniglide compatible).