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[Heimdall]

You don't need to downgrade your dash, you need to upgrade it - to 5960. Any "recent" live enabled game (Halo 2 or later) will allow you to do that - see this post.

Alternatively you can get a stock 5960 dash from CASH or Slayers (use Qwix to extract it from the iso), trigger the Krayzie exploit, then use ftp to delete the current C:\ contents and replace them with the 5960 dash. Once you've done that the basic install will work.