Yes, you will be able to. Normally the Parking Lot will open at 7:00 and the monorail will be running at that time. Usually only the Resorts monorail will run that early, not the Express.
For an 8:00 reservation you should figure on being at the toll plaza no later than 7:15 to allow time to park, get to the TTC, on the monorail, and into the park.