However you go, if indeed 7 or 8 speed, throw on a 9spd (4400) tiagra long cage derailluer. They're low bucks and the chain will shift quick and smooth. I did a bit of 7-speed non-related cassette cog swapping a while back that ended as thus:

a fresh 13-23 w/HG chain and had a 9-speed 14 cog and an 8-speed 18 cog and it all worked in an adequate fashion (filing to get the ramps close). After a while I decided to try the Tiagra I had laying about and the fool thing improved a good deal.

I've since returned to 9-speed but I rode that frankenstein smallblock for alotta miles.