You're not going to be able to get enough premium in there with a tank already full of regular and a couple gallons of toluene. Don't worry about topping it off if it's already quite full. Actually, it'll mix better if there's a little room for it to splash around. Worst case scenario is that you end up running 105 octane for a a couple hundred miles.
How much fuel is in the tank? I had assumed 3/4 or more. If it's less you'll want to adjust how much toluene you add accordingly.