webman1,
I'm afraid I dont know specifically which pages will require hacking to make that into a county field. You could try doing a search through the forums for this, its been a while since I saw anything about it.

As an alternative, you may want to look into the option of setting up counties as zones instead of hacking the pages.
There are directions in the readme file for the Local Sales Tax mod on how do set up each county as a zone.

LadyHLG