Hey Tim, I'm in Miami also, but expect to be heading out within a week or so (construction project down here over the past few months winding down). Regarding your question, I have the same 258 and with the Carter carb and no emissions, the best I could squeeze was about 12-13 mpg around town, and 15 on the highway. Upgrading to the weber got me around 15 in the city and 18 on the highway, although with the freeway traffic down here it suffers! You'll appreciate the power of the weber, especially when you floor it and the secondary kicks in. Check this site www.weber-carburetors.com/after.htm
for some deals on the K551 if you're interested.
Otherwise, not much you can do. I run my tires at 30 psi to minimize resistance, but that's all I can think of.