THis is not a Logic 8 issue, it's a common problem with Loigc Suite's original disks. You must call Apple and they will give you a free set of replacement disks (Disk 1 and 2 I believe)
Your other option is to use a program like Pacifist to force an install and then run the Update to 9.1.8 from the web (not software update), this should replace the PowerPC parts of your Logic install that are only compatible with OSX 10.6 and below.
Maybe for you, but for me 9.1.8 is the only reason I still use Logic. It cleared up all the awful lag bugs in Lion and reduced the Logic.app filesize to about a quarter of what it used to be. To me thats one big weight loss, and weight losses are a good thing when it comes to programs.
You may have been experienceing a delta upgrade issue yourself. That's why I suggested running the update fromt the downloaded installer. That deletes your files and replaces them with the new ones rather than the delata method which literally rewrites portions of your existing files with the intention to make them into the new files. Delat upgrades are smaller downloads but Apple has shown that they can cause ugly bugs.