"Best" rpm for a stovebolt would be where your particular engine is putting out maximum torque, based on cam grind, displacement etc, (generally around 2500 rpm). Stock rear is 4.11, the 3.55 set is the only aftermarket replacement for the torque tube.
http://www.stovebolt.com/ubbthreads/...&Number=340589
The I-15's around Vegas are fast freeways, at "only" 65 mph I would be a moving roadblock. Many of us replace the torque tube with an open differential, I'm running a 3.08 rear which allows me to reasonably keep up with Calif freeway speeds. Of course you have a really nice original truck which you probably want to keep stock, so 3.55 gears are your best bet.