If I backup C:40GB to ExBoot and then replaced internal drive to 80GB. How do I ensure the restore is using the full 80GB?

This is possible if you can boot up from USB hard disk and then do restoration. When boot up from USB hard disk, first go to Disk Management and format the new 80GB internal disk as a single drive. Next, launch ExBoot software and start restoration. It will recognize the newly formatted internal drive as the destination.