I also do alot of highway driving. The 4.10's do not bother me on the freeway at about 70 I am still under 3000 rpm's. My fifth gear is actually usable now I can pass people now problem in fifth. With 3.27's I couldn't pass nothing without gearing down.
A $1000 is way to high to put gears in a car. It should cost about 500-600.
Check this out about recalibrating you speedometer.
http://www.dallasmustang.com/earlyspecials.htm
Hope this helps!!
Don't let this shop screw you out of money. Go somewhere else.