I think the big problem isn't tank size, it's tank weight.
Water weighs 8.35 pounds per gallon. Even just a 25 gallon increase at the back of a long overhang could result in a huge difference in the axle weight and balance.
Most 350/3500 series C's are at or really close to GVWR when they roll out of the factory. Even the E-450 chassis under a 30' unit doesn't have a lot of CCC left.
Before I changed a tank I'd load up the existing one and run it over a scale.