Sorry I made some errors at my previous post (I was sleepy).
This procedure should be correct.
1. Copy a KDE 3.4.2 complete tree structure [with files] from a source section of the ftp.slackware.com site. Remove older KDE342 *.tar.bz2 source files from subdirectories.
2. Insert every KDE 351 source file [from kde.org] into appr. subdirectory.
3. Change the KDE version number to 3.5.1 inside the top-level option file named "KDE.options". Do it that way in all subdirectories containing "local.options" file via editing that file. Version number can be different (Kdevelop , Koffice) but should match appropiate source KDE component package's version number.
4. Delete all files in subdirs with "diff" or "patch" in its name - we won't be patching anything now.
5. Make sure all scripts in top- and subdirs are executable /adjust file permissions when needed/
6. Make sure you have at least 3 GB of Hdd free space and appr. 20+ hours to compile KDE.
7. Log out of Kde and X -> relogin as root -> removepkg your older KDE component packages except Qt package.
8. Run main KDE.SlackBuild script. Watch the screen while processing - if errors occurs something must be corrected... I had too small Hdd space...
9. Go somewhere ... for sleep for example.
10. Happy end of this process results in NEW fresh KDE tgz packages sitting in /tmp directory. The new packages are actually installed by Slackbuild scripts.
That worked for me while compiling Kde 350.
BUT I cannot guarantee I remember everything perfectly....do it on your own risk :-)
Remember to remove all unneeded source files [in /tmp] except your fresh TGZ packages !