Overdrive options

i have a prostreet duster, 340 ,reverse shift 727 transmission and 33" MT's. The 9" ford has 4:86 gears. I'm turning around 3500 at @60. This is making it painfull to travel any distance on the interstate.
I'd like to keep the gearing, so I'm considering an overdrive. I've looked at the gear vendors but maybe you guys have other suggestions.