You forgot a few key points...

SLOW DOWN ON THE FREEWAY. You have a 6500lbs+ brick, it's not made to haulass at 85mph. If you need to, get a highway cruiser to do that.

The truck has donut sized stock tires (~245/75R16) and with the .75:1 OD and a 4.10:1 it'll tach out like crazy even at 70mph. So stick to 65mph max to get some economy from it.

Now, how large are the tires on it currently? You said they are wider and taller... a 33" AT shouldn't be too bad at all. Should still be well over 2000rpm @ 70mph.

After that do a few things to make it more efficient. A tune works nicely, intake, some full length headers too. Make it the most efficient it can, but I still stress don't drive so damned fast in it.