Now I know you fancy PSD guys like overdrives like gear vendors...but bear with me.
I know of one guy who went through putting a brownie box in his truck. Basically one of those old style underdrive/direct/overdrive gearboxes found on medium duty trucks back in the day. I forget the model of his, but its a underdrive/direct/overdrive model. Pulled the milage up a couple. Towing his 5'er he gets around 14.5-15mpg in his IDI. Not as fancy as those gear vendors or US overdrives, but they get the job done and definatly give you some gear selection choices. Only downside is a lot of fabbing involved, not for the faint of heart.