It will just be enough. When you pour it in, do cover the fill hole with tape to slow the curing down while you tumble the tank to get a full coating. If you use the small batch, I'd fill through the petcock first to insure you coat the underside of the top. It will drip down to help coat the floor later.
If your tank has crossover tubes, stuff a wire (just large enough) in from below to prevent the liner from clogging these. Twist them every 5 minutes while you tumble, then keep them loose during the first 30 minutes of the curing process, and remove before the liner sets. And you should be good to go-