Lots of versions of the 600 freehub, some were Uniglide, and Hyperglide. I replaced the 600 Uniglide on a wheel I had, with a donor hyperglide freehub. That can sometimes work (they also had a variety of mounting designs).

Short answer, without pulling the freehub, no idea. 7 speed cassettes are readily available, at low cost.

A lot of shops around here have no understanding of older bikes, and have told me everything I have is obsolete. Kind of a standard answer really, if they don't know, "its obsolete, you can't get them anymore".

Spend some time on the Sheldon Brown site, it goes through it thoroughly. In addition, there was a post on here a couple of weeks ago with the oddball freehub attachment.