Nikon scanners were made to quite a high degree of uniformity. so before getting too concerned about the unavailability of a Kodachrome target for Auto-IT8 profiling, you should try using the canned Kodachrome profile for that scanner which comes bundled with your version of SilverFast 8. In fact, when you select "Kodachrome" in the second from the left "icon" of the media selection panel, the colour management preferences should automatically load the correct profile. You can verify this by going to Preferences>CMS>Profiles>Input ("Input" is SilverFastese for "Scanner Profile"), where the scanner profile <SFProfK(LS-5000 ED).icc> should show. If it doesn't, select it from the list (appears when you click on the pane) and try using it to see what quality scan you get from your Kodachrome slides.