Well, the final drive on the 4 speed's fourth gear and the 3 speed's third gear are the same. So, freeway rpm's will be the same as yours was. The advantage of a 4 speed is in the better spaced shift points.
My truck is about like yours. Slightly lowered, dual exhaust and this summer dual carbs. Otherwise it will be mainly stock.
If mine didn't shift so nice the 4 speed would go in for sure.
