Looks like you have 3 more brackets you could use too. Your bike looks made for the road!I don't ride with a passenger, so I built a bracket across the rear seat. for the 1 gallon.
Sorry. I don't have a pic without the stupid bungee net.
There are many who simply bolt the 1.75 gallon to the back rack. It makes a good base for soft luggage.
I wear a hydration pack w/ 100 ounces of water, AND I carry a "nalgene" water bottle (the markings have worn off, but I think it holds two L...) in the bag holding my tent. That gets used for my breakfast water, and I ALWAYS fill it right after it's used, so IF I camp where there's NO water, I'm still fine.... I may need a 1 gallon container for water too....