The KDE bugzilla has one pretty obvious int32 -> int64 patch. I am positive that it's at least part of the solution. I searched the bugzilla archives and found a few reports on data corruptions on large files, most of them marked duplicates for each other.

The problem under Debian is that you can update to the testing version of ktorrent, but not without updating most of the KDE packages, too.

I submitted a Debian bug report, it's bug #448666.