If you have a half ton, you probably have 4.11 gears (at best 3.73s).
I could ride at 55-60 all day with my 3.73 gears when I had the stock 3 speed transmission. 65 was pushing it, for extended driving. And the absolute top speed was around 80-85.
If you plan on doing a lot of highway cruising then you need to change to either an overdrive transmission or a 3.07 (or numerically lower) rear end. I think the overdrive transmission is the best solution. You keep the same rear end, have better low end gearing for "off the line" acceleration" and end up (usually) with lower cruising rpms on the top end.
-ftyler
