So iTunes 11.1 seems to kill Custom IPSWs for iOS 7. Alternatives will be needed to perform custom restores.
(More technically, iTunes 11.1 recalculates the partial-hash of every img3 instead of using the ones in BuildManifest.plist). Which results in modified images having a different hash and being rejected by Apple’s TSS.
In other news, iTunes 11.1 also breaks iREB on s5l8900x devices (iPhone 2G, iPhone 3G, and iPod touch 1G).
Thankfully, it appears as though iH8Sn0w is already working on a tool to restore without iTunes. You can follow iClarified on Twitter, Facebook, Google+, or RSS for updates.