I would recommend you cut the top off the tank, use fiberglass to build up the slope toward the drain, and then glass the top back on.
On mine the top of the tank was so sloped that I could not get the toilet to sit flat. The toilet was at least 1" higher on one side. So I cut the top off so I could put it back on level.
Also, the fiberglas around the outlet flange was cracked and needed to be repaired. While I was at it I added a cleanout hole on the top, and added a vent to replace the original vent, which was vulkemed into the back of the toilet.
Good luck whatever you do. If you want to see pics look on my page.