Will the script work for 100% ? Probably not, it depends on if the patches can be applied succesfully and changes to the X packages. But you could try the Slackware current source which is X.org 6.8.1 and compile those on your Slackware 10 .
Font packages are built as well if I look to the script
The script does not install, it creates packages in /tmp
Forgot your last question: You could do both, it makes no difference. Upgradepkg is less work I would say
