ATS and Quafe make nice gear sets, ussualy it's the whole 3,4, and 5th gears, should run you about 650-900 bucks, but what you're describing is a lack of gearing up high, not a problem. You're trying to push a 2500lb car in 5th gear against a lot of wind. It's gonna take a while to get up there, either change gears, or deal with it.

Later,
Aj