[Overdriven]

I'd have a look at pics of stock height trucks and note where the axle centerline is compared to the body. This will help you determine what springs to use and if you need a lift kit or not.

As far as feeling floaty on the highway that's a function of the shocks not the springs. The reason it feels floaty is because after the suspension compresses it is uncompressing (rebound) too fast resulting in the suspension cycling back and forth between compression and rebound instead of coming back to its resting postition. Picture one of those spring loaded door stops that keep going back and forth after you bend it and let it go. The door stop is an extreme example but I'm sure you've seen vehicles that seem to continually bounce up and down on the road after hitting a bump. The shock is supposed to slow this movement (by absorbing the springs energy) so the suspension returns to resting position quicker instead of cycling between compression and rebound which feels floaty. Your current shocks could be worn out or just not have enough rebound dampening.