Speed and handling with an early truck are almost like oil and water, they just don't go together. (Unless you're BP) With a straight axle, leaf sprung truck, I would go more for the comfort side of things. I can't think that moving up to 19" rim would help handling much at all, especially if you're using a LT tire anyway. Most modern Dually trucks are still using 16" and 17" tires and they have more towing and load capabilities than any truck did in the past. I'd stick with 16" or 17".
