I got mine from Ford for about $70 and did it when I installed my new gears. If you're not doing a gear change, it should be a relatively easy job. The hardest part is inserting the spring.
This article covers what's involved to do a complete gear swap, but it also has info on doing the clutch packs.
8.8 Gear Install
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351W 89 Mustang GT Convertible
[This message has been edited by jimberg (edited 10-26-2001).]