Joined: Jan 2010
Posts: 9,834
Likes: 1,811
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.
+1
The UG cassette cogs are among the very few that can be reversed without shifting problems.
Alternately, 13-30t and 13-34t, 7-speed HG cassettes are still available. You'd only need a new derailer and longer chain, and your crankset would likely be fine as-is. A 13t threaded cog would replace the splined cog of the new cassette, with almost no chance of chain contacting frame even using the 6sp-spaced version.
GonzoBob is right, the biggest tab on each cog needs to be filed a little after the cogs are separated, but I've also seen the whole cassette filed out at once (held in a vise, using a bigger file).