I'm currently plotting a DD and had some questions about rear end gearing to suit my driving habits. Without changing the factory 250 and 4 speed transmission, how tall can I go (practically speaking) with rear end gearing?

A typical week will have me driving about 300 miles highway (60 mph), 50 miles in moderate-to-heavy in-town traffic, and about 200 miles on the Interstate. I know the Interstate travel is going to remain problematic, but I'd like to have something better for the highway driving than the 4.10 that was in my last '66.

Thanks!
Jim